Popular Science Monthly/Volume 24/November 1883/Some Unsolved Problems in Geology II
|←The Age of Trees|| Popular Science Monthly Volume 24 November 1883 (1883)
Some Unsolved Problems in Geology II
By John William Dawson
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By Dr. J. W. DAWSON.
AGAIN: we are now prepared to say that the struggle for existence, however plausible as a theory, when put before us in connection with the productiveness of animals, and the few survivors of their multitudinous progeny, has not been the determining cause of the introduction of new species. The periods of rapid introduction of new forms of marine life were not periods of struggle, but of expansion—those periods in which the submergence of continents afforded new and large space for their extension and comfortable subsistence. In like manner it was continental emergence that afforded the opportunity for the introduction of land animals and plants. Further, in connection with this, it is now an established conclusion that the great aggressive faunas and floras of the continent have originated in the north, some of them within the Arctic Circle; and this in periods of exceptional warmth, when the perpetual summer sunshine of the Arctic regions coexisted with a warm temperature. The testimony of the rocks thus is, that not struggle, but expansion, furnished the requisite conditions for new forms of life, and that the periods of struggle were characterized by depauperation and extinction.
But we are sometimes told that organisms are merely mechanical, and that the discussions respecting their origin have no significance, any more than if they related to rocks or crystals, because they relate merely to the organism considered as a machine, and not to that which may be supposed to be more important, namely, the great determining power of mind and will. That this is a mere evasion, by which we really gain nothing, will appear from a characteristic extract of an article by an eminent biologist, in the new edition of the "Encyclopædia Britannica"—a publication which, I am sorry to say, instead of its proper róle as a repertory of facts, has become a strong partisan, stating extreme and unproved speculations as if they were conclusions of science. The statement referred to is as follows: "A mass of living protoplasm is simply a molecular machine of great complexity, the total results of the working of which, or its vital phenomena, 62 THE POPULAR SCIENCE MONTHLY.
depend on the one hand on its construction, and, on the other, on the energy supplied to it ; and to speak of vitality as anything but the name for a series of operations is as if one should talk of the horo- logity of a clock." It would, I think, scarcely be possible to put into the same number of words a greater amount of unscientific assumption and unproved statement than in this sentence. Is "living proto- plasm " different in any way from dead protoplasm, and, if so, what causes the difference ? What is a " machine " ? Can we conceive of a self -produced or uncaused machine, or one not intended to work out some definite results? The results of the machine in question are said to be " vital phenomena " ; certainly most wonderful results, and greater than those of any machine man has yet been able to construct ! But why " vital " ? If there is no such thing as life, surely they are merely physical results. Can mechanical causes produce other than physical effects ? To Aristotle, life was " the cause of form in organ- isms." Is not this quite as likely to be true as the converse proposir tion ? If the vital phenomena depend on the " construction " of the machine, and the " energy supplied to it," whence this construction, and whence this energy ? The illustration of the clock does not help us to answer this question. The construction of the clock depends on its maker, and its energy is derived from the hand that winds it up. If we can think of a clock which no one has made and which no one winds a clock constructed by chance, set in harmony with the uni- verse by chance, wound up periodically by chance we shall then have an idea parallel to that of an organism living, yet without any vital energy or creative law ; but in such a case we should certainly have to assume some antecedent cause, whether we call it " horologi- ty " or by some other name. Perhaps the term " evolution " would serve as well as any other, were it not that common sense teaches that nothing can be spontaneously evolved out of that in which it did not previously exist.
There is one other unsolved problem, in the study of life by the geologist, to which it is still necessary to advert. This is the inability of paleontology to fill up the gaps in the chain of being. In this re- spect, we are constantly taunted with the imperfection of the record ; but facts show that this is much more complete than is generally sup- posed. Over long periods of time and many lines of being, we have a nearly continuous chain ; and, if this does not show the tendency desired, the fault is as likely to be in the theory as in the record. On the other hand, the abrupt and simultaneous appearance of new types in many specific and generic forms, and over wide and separate areas at one and the same time, is too often repeated to be accidental. Hence paleontologists, in endeavoring to establish evolution, have been obliged to assume periods of exceptional activity in the introduction of species, alternating with others of stagnation a doctrine differing very little from that of special creation as held by the older geologists.
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The attempt has lately been made to account for these breaks by the assumption that the geological record relates only to periods of submergence, and gives no information as to those of elevation. This is manifestly untrue. In so far as marine life is concerned, the periods of submergence are those in which new forms abound for very obvi- ous reasons already hinted. But the periods of new forms of land and fresh-water life are those of elevation, and these have their own records and monuments, often very rich and ample ; as, for example, the swamps of the carboniferous, the transition from the cretaceous subsidence to the Laramie elevation, the tertiary lake-basins of the West, the terraces and raised beaches of the pleistocene. Had I time to refer in detail to the breaks in the continuity of life, which can not be explained by the imperfection of the record, I could show at least that nature, in this case, does advance per saltum by leaps, rather than by a slow, continuous process. Many able reasoners, as Le Conte in this country, and Mivart and Collard in England, hold this view.
Here, as elsewhere, a vast amount of steady conscientious work is required to enable us to solve the problems of the history of life. But, if so, the more the hope for the patient student and investigator. I know nothing more chilling to research, or unfavorable to progress, than the promulgation of a dogmatic decision that there is nothing to be learned but a merely fortuitous and uncaused succession, amenable to no law, and only to be covered, in order to hide its shapeless and uncertain proportions, by the mantle of bold and gratuitous hypothesis.
So soon as we find evidence of continents and oceans, we raise the question, " Have these continents existed from the first in their pres- ent position and form, or have the land and water changed places in the course of geological time ? " In reality both statements are true in a certain limited sense. On the one hand, any geological map whatever suffices to show that the general outline of the existing land began to be formed in the first and oldest crumplings of the crust. On the other hand, the greater part of the surface of the land consists of marine sediments which must have been derived from land that has perished in the process, while all the continental surfaces, except, perhaps, some high peaks and ridges, have been many times sub- merged. Both of these apparently contradictory statements are true ; and, without assuming both, it is impossible to explain the existing contours and reliefs of the surface.
In the case of North America, the form of the old nucleus of Lau- rentian rock in the north already marks out that of the finished conti- nent, and the successive later formations have been laid upon the edges of this, like the successive loads of earth dumped over an embankment. But, in order to give the great thickness of the palaeozoic sediments, the land must have been again and again submerged, and for long periods of time. Thus, in one sense, the continents have been fixed ;
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in another, they have been constantly fluctuating. Hall and Dana have well illustrated these points in so far as Eastern North America is concerned. Professor Hull, of the Geological Survey of Ireland, has recently had the boldness to reduce the fluctuations of land and water, as evidenced in the British Islands, to the form of a series of maps intended to show the physical geography of each successive period. The attempt is probably premature, and has been met with much ad- verse criticism ; but there can be no doubt that it has an element of truth. When we attempt to calculate what could have been supplied from the old eozoic nucleus by decay and aqueous erosion, and when we take into account the greater local thickness of sediments toward the present sea-basins, we can scarcely avoid the conclusion that ex- tensive areas once occupied by high land are now under the sea. But to ascertain the precise areas and position of these perished lands may now be impossible.
In point of fact, we are obliged to believe in the contemporaneous existence in all geological periods, except perhaps the very oldest, of three sorts of areas on the surface of the earth : 1. Oceanic areas of deep sea, which must always have occupied the bed of the present ocean, or parts of it ; 2. Continental plateaus, sometimes existing as low flats or as higher table-lands, and sometimes submerged ; 3. Areas of plication or folding, more especially along the borders of the oceans, forming elevated lands rarely submerged, and constantly affording the material of sedimentary accumulations.
Every geologist knows the contention which has been occasioned by the attempts to correlate the earlier palaeozoic deposits of the At- lantic margin of North America with those forming at the same time on the interior plateau, and with those of intervening lines of plication and igneous disturbance. Stratigraphy, lithology, and fossils are all more or less at fault in dealing with these questions ; and, while the general nature of the problem is understood by many geologists, its solution in particular cases is still a source of apparently endless de- bate.
The causes and mode of operation of the great movements of the earth's crust which have produced mountains, plains, and table-lands, are still involved in some mystery. One patent cause is the unequal settling of the crust toward the center ; but it is not so generally un- derstood as it should be that the greater settlement of the ocean-bed has necessitated its pressure against the sides of the continents in the same manner that a huge ice-floe crushes a ship or a pier. The geo- logical map of North America shows this at a glance, and impresses us with the fact that large portions of the earth's crust have not only been folded, but bodily pushed back for great distances. On looking at the extreme north, we see that the great Laurentian mass of cen- tral Newfoundland has acted as a protecting pier to the space imme- diately west of it, and has caused the Gulf of St. Lawrence to re-
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main an undisturbed area since palaeozoic times. Immediately to the south of this, Nova Scotia and New Brunswick are folded back. Still farther south, as Guyot has shown, the old sediments have been crushed in sharp folds against the Adirondack mass, which has shel- tered the table-land of the Catskills and of the Great Lakes. South of this, again, the rocks of Pennsylvania and Maryland have been driven back in a great curve to the west. Nothing, I think, can more forci- bly show the enormous pressure to which the edges of the continents have been exposed, and at the same time the great sinking of the ocean-beds. Complex and difficult to calculate though these move- ments of plication are, they are more intelligible than the apparently regular pulsations of the flat continental areas, whereby they have alternately been below and above the waters, and which must have depended on somewhat regularly recurring causes, connected either with the secular cooling of the earth, or with the gradual retardation of its rotation, or with both. Throughout these changes, each succes- sive elevation exposed the rocks for long ages to the decomposing in- fluence of the atmosphere. Each submergence swept away, and de- posited as sediment, the material accumulated by decay. Every change of elevation was accompanied with changes of climate and with modifications of the habitats of animals and plants. Were it possible to restore accurately the physical geography of the earth in all these respects, for each geological period, the data for the solution of many difficult questions would be furnished.
It is an unfortunate circumstance that conclusions in geology, ar- rived at by the most careful observation and induction, do not remain undisturbed, but require constant vigilance to prevent them from being overthrown. Sometimes, of course, this arises from new discoveries throwing new light on old facts ; but when this occurs it rarely works the complete subversion of previously received views. The more usual case is, that some over-zealous specialist suddenly discovers what seems to him to overturn all previous beliefs, and rushes into print with a new and plausible theory, which at once carries with him a host of half -informed people, but the insufficiency of which is speedily made manifest.
Had I written this address a few years ago, I might have referred to the mode of formation of coal as one of the things most surely set- tled and understood. The labors of many eminent geologists, micro- scopists, and chemists in the Old and the New Worlds had shown that coal nearly always rests upon old-soil surfaces penetrated with roots, and that coal-beds have in their roofs erect trees, the remains of the last forests that grew upon them. Logan and I have illustrated this in the case of the series of more than sixty successive coal-beds exposed at the South Joggins, and have shown unequivocal evidence of land- surfaces at the time of the deposition of the coal. Microscopical ex- amination has proved that these coals are composed of the materials
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of the same trees whose roots are found in the under-clays, and their stems and leaves in the roof -shales ; that much of the material of the coal has been subjected to sub-aerial decay at the time of its accumula- tion ; and that in this, ordinary coal differs from bituminous shale, earthy bitumen, and some kinds of cannel, which have been formed under water ; that the matter remaining as coal consists almost en- tirely of epidermal tissues, which, being suberose in character, are highly carbonaceous, very durable, and impermeable by water,* and are hence the best fitted for the production of pure coal ; and finally that the vegetation and the climatal and geographical features of the coal period were eminently fitted to produce in the vast swamps of that period precisely the effects observed. All these points and many others have been thoroughly worked out for both European and Ameri- can coal-fields, and seemed to leave no doubt on the subject. But sev- eral years ago certain microscopists observed on slices of coal layers filled with spore-cases a not unusual circumstance, since these were shed in vast abundance by the trees of the coal-forests, and because they contain suberose matter of the same character with epidermal tissues generally. Immediately we were informed that all coal con- sists of spores ; and, this being at once accepted by the unthinking, the results of the labors of many years are thrown aside in favor of this crude and partial theory. A little later, a German microscopist has thought proper to describe coal as made up of minute algae, and tries to reconcile this view with the appearances, devising at the same time a new and formidable nomenclature of generic and specific names, which would seem largely to represent mere fragments of tissues. Still later, some local facts in a French coal-field have induced an emi- nent botanist of that country to revive the drift theory of coal, in op- position to that of growth in situ. A year or two ago, when my friend Professor Williamson, of Manchester, informed me that he was pre- paring a large series of slices of coal with the view of revising the whole subject, I was inclined to say that, after what had been done by Lyell, Goeppert, Logan, Hunt, Newberry, and myself, this was scarcely necessary ; but, in view of what I have just stated, it may be that all he can do will be required to rescue from total ruin the results of our labors.
An illustration of a different character is afforded by the contro- versy now raging with respect to the so-called fucoids of the ancient rocks. At one time the group of fucoids, or algae, constituted a gen- eral place of refuge for all sorts of unintelligible forms and markings ; graptolites, worm-trails, crustacean tracks, shrinkage-cracks, and, above all, rill-markings, forming a heterogeneous group of fucoidal remains distinguished by generic and specific names. To these were also added some true land-plants badly preserved, or exhibiting structures not well understood by botanists. Such a group was sure to be eventually
- " Acadian Geology," third edition, supplement, p. 68.
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dismembered. The writer has himself done something toward this * but Professor Nathorst has done still more : f and now some intellio-i- ble explanation can be given of many of these forms. Quite recently, however, the Count de Saporta, in an elaborate illustrated memoir,]; has come to the defense of the fucoids, more especially against the destructive experiments of Nathorst, and would carry back into the vegetable kingdom many things which would seem to be mere trails of animals. While writing this address, I have received from Pro- fessor Crie, of Rennes, a paper in which he not only supports the algal nature of rusichnites, arthrichnites, and many other supposed fucoids, but claims for the vegetable kingdom even receptaculites and archaeo- cyathus. It is not to be denied that some of the facts which he cites, respecting the structure of the siphoniae and of certain modern incrust- ing alga?, are very suggestive, though I can not agree with his conclu- sions. My own experience has convinced me that, while non-botanical geologists are prone to mistake all kinds of markings for plants, even good botanists, when not familiar with the chemical and mechanical conditions of fossilization, and with the present phenomena of tidal shores, are quite as easily misled, though they are very prone, on the other hand, to regard land-plants of some complexity, when badly pre- served, as mere algae. In these circumstances it is very difficult to secure any consensus, and the truth is only to be found by careful observation of competent men. One trouble is, that these usually obscure markings have been despised by the greater number of paleon- tologists, and probably would not now be so much in controversy were it not for the use made of them in illustrating supposed phylogenies of plants.
It would be wrong to close this address without some reference to that which is the veritable pons asinorum of the science, the great and much-debated glacial period. I trust that you will not suppose that, in the end of an hour's address, I am about to discuss this vexed question. Time would fail me even to name the hosts of recent au- thors who have contended in this arena. I can hope only to point out a few landmarks which may aid the geological adventurer in travers- ing the slippery and treacherous surface of the hypothetical ice-sheet of pleistocene times, and in avoiding the yawning crevasses by which it is traversed.
No conclusions of geology seem more certain than that great changes of climate have occurred in the course of geological time ; and the evidence of this in that comparatively modern period which immediately preceded the human age is so striking that it has come to be known as pre-eminently the ice age, while, in the preceding ter-
- "Footprints and Impressions on Carboniferous Rocks," "American Journal of Sci-
f Royal Swedish Academy, Stockholm, 1881. \ "A propos des Algues Fossiles," Paris, 1883.
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tiary periods, temperate conditions seem to have prevailed even to the pole. Of the many theories as to these changes which have been proposed, two seem at present to divide the suffrages of geologists, either alone, or combined with each other. These are 1. The theory of the precession of the equinoxes in connection with the varying ec- centricity of the earth's orbit, advocated more especially by CrolL; and, 2. The different distribution of land and water as affecting the reception and radiation of heat and the ocean-currents a theory ably propounded by Lyell, and subsequently extensively adopted, either alone or with the previous one. One of these views may be called the astronomical ; the other, the geographical. I confess that I am in- clined to accept the second or Lyellian theory, for such reasons as the following : 1. Great elevations and depressions of land have occurred in and since the pleistocene, while the alleged astronomical changes are not certain, more especially in regard to their probable effect on the earth. 2. "When the rival theories are tested by the present phe- nomena of the southern polar region and the North Atlantic, there seem to be geographical causes adequate to account for all except ex- treme and unproved glacial conditions. 3. The astronomical cause would suppose regularly recurring glacial periods of which there is no evidence, and it would give to the latest glacial age an antiquity which seems at variance with all other facts. 4. In those more northern re- gions where glacial phenomena are most pronounced, the theory of floating sheets of ice, with local glaciers descending to the sea, seems to meet all the conditions of the case ; and these would be obtained, in the North Atlantic at least, by very moderate changes of level, caus- ing, for example, the equatorial current to flow into the Pacific, instead of running northward as a gulf stream. 5. The geographical theory allows the supposition not merely of vicissitudes of climate quickly following each other in unison with the movements of the surface, but allows also of that near local approximation of regions wholly covered with ice and snow, and others comparatively temperate, which we see at present in the north.
If, however, we are to adopt the geographical theory, we must avoid extreme views ; and this leads to the inquiry as to the evidence to be found for any such universal and extreme glaciation as is de- manded by some geologists.
The only large continental area in the northern hemisphere sup- posed to be entirely ice- and snow-clad is Greenland ; and this, so far as it goes, is certainly a local case, for the ice and snow of Greenland extend to the south as far as G0 north latitude, while both in Norway and in the interior of North America the climate in that latitude per- mits the growth of cereals. Further, Grinnell Land, which is separated from North Greenland only by a narrow sound, has a comparatively mild climate, and, as Nares has shown, is covered with verdure in summer. Still further, Nordenskiold, one of the most experienced
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Arctic explorers, holds that it is probable that the interior of Green- land is itself verdant in summer, and is at this moment preparing to attempt to reach this interior oasis. Nor is it difficult, with the aid of the facts cited by Woeickoff and Whitney,* to perceive the cause of the exceptional condition of Greenland. To give ice and snow in large quantities, two conditions are required first, atmospheric hu- midity ; and, secondly, cold precipitating regions. Both of these conditions meet in Greenland. Its high coast-ranges receive and con- dense the humidity from the sea on both sides of it and to the south. Hence the vast accumulation of its coast snow-fields, and the intense discharge of the glaciers emptying out of its valleys. When extreme glacialists point to Greenland, and ask us to believe that in the glacial age the whole continent of North America as far south as the latitude of 40 was covered with a continental glacier, in some places several thousands of feet thick, we may well ask, first, what evidence there is that Greenland, or even the Antarctic Continent, at present shows such a condition ; and, secondly, whether there exists a possibility that the interior of a great continent could ever receive so large an amount of precipitation as that required. So far as present knowledge exists, it is certain that the meteorologist and the physicist must answer both questions in the negative. In short, perpetual snow and glaciers must be local, and can not be continental, because of the vast amount of evaporation and condensation required. These can only be possible where comparatively warm seas supply moisture to cold and elevated land ; and this supply can not, in the nature of things, penetrate far inland. The actual condition of interior Asia and interior America in the higher northern latitudes affords positive proof of this. In a state of partial submergence of our northern continents, we can readily imagine glaciation by the combined action of local glaciers and great ice-floes ; but, in whatever way the phenomena of the bowlder clay and of the so-called terminal moraines are to be accounted for, the theory of a continuous continental glacier must be given up.
I can not better indicate the general bearing of facts, as they pre- sent themselves to my mind in connection with this subject, than by referring to a paper by Dr. G. M. Dawson on the distribution of drift over the great Canadian plains east of the Rocky Mountains. f I am the more inclined to refer to this, because of its recency, and because I have so often repeated similar conclusions as to Eastern Canada and the region of the Great Lakes.
The great interior plain of Western Canada, between the Lauren- tian axis on the east and the Rocky Mountains on the west, is seven hundred miles in breadth, and is covered with glacial drift, presenting one of the greatest examples of this deposit in the world. Proceed-
- " Memoir on Glaciers," Geological Society of Berlin, 1881; "Climatic Changes,"
\ " Science," July 1, 1883.
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ing eastward from the base of the Rocky Mountains, the surface, at first more than four thousand feet above the sea-level, descends by- successive steps to twenty-five hundred feet, and is based on creta- ceous and Laramie rocks, covered by bowlder clay and sand, in some places from one hundred to two hundred feet in depth, and filling up pre-existing hollows, though itself sometimes piled into ridges. Near the Rocky Mountains the bottom of the drift consists of gravel not glaciated. This extends to about one hundred miles east of the moun- tains, and must have been swept by water out of their valleys. The bowlder clay resting on this deposit is largely made up of local debris in so far as its paste is concerned. It contains many glaciated bowl- ders and stones from the Laurentian region to the east, and also smaller pebbles from the Rocky Mountains ; so that at the time of its forma- tion there must have been driftage of large stones for seven hundred miles or more from the east, and of smaller stones from a less distance on the west. The former kind of material extends to the base of the mountains, and to a height of more than four thousand feet. One bowlder is mentioned as being forty-two by forty by twenty feet in dimensions. The highest Laurentian bowlders seen w r ere at an eleva- tion of forty-six hundred and sixty feet, on the base of the Rocky Mountains. The bowlder clay, when thick, can be seen to be rudely stratified, and at one place includes beds of laminated clay with com- pressed peat, similar to the forest-beds described by Worthen and Andrews in Illinois, and the so-called interglacial beds described by Hinde on Lake Ontario. The leaf -beds on the Ottawa River and the drift-trunks found in the bowlder clay of Manitoba belong to the same category, and indicate that throughout the glacial period there were many forest oases far to the north. In the valleys of the Rocky Mountains opening on these plains there are evidences of large local glaciers now extinct, and similar evidences exist on the Laurentian highlands on the east.
Perhaps the most remarkable feature of the region is that immense series of ridges of drift piled against an escarpment of Laramie and cretaceous rocks, at an elevation of about twenty-five hundred feet, and known as the " Missouri Coteau." It is in some places thirty miles broad and a hundred and eighty feet in height above the plain at its foot, and extends north and south for a great distance ; being, in fact, the northern extension of those great ridges of drift which have been traced south of the Great Lakes, and through Pennsylvania and New Jersey, and which figure on the geological maps as the edge of the continental glacier an explanation obviously inapplicable in those Western regions where they attain their greatest development. It is plain that in the North it marks the western limit of the deep water of a glacial sea, which at some periods extended much farther west, perhaps with a greater proportionate depression in going west- ward, and on which heavy ice from the Laurentian districts on the
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east was wafted southwestward by the Arctic currents, while lighter ice from the Rocky Mountains was being borne eastward from these mountains by the prevailing westerly winds. We thus have in the West, on a very wide scale, the same phenomena of varying submer- gence, cold currents, great ice-floes, and local glaciers producing ice- bergs, to which I have attributed the bowlder clay and upper bowl- der drift of Eastern Canada.
A few subsidiary points I may be pardoned for mentioning here. The rival theories of the glacial period are often characterized as those of land glaciation and sea-borne icebergs. But it must be remem- bered that those who reject the idea of a continental glacier hold to the existence of local glaciers on the highlands more or less exten- sive during different portions of the great pleistocene submergence. They also believe in the extension of these glaciers seaward and partly water-borne, in the manner so well explained by Mattieu Will- iams ; in the existence of those vast floes and fields of current-and tide-borne ice whose powers of transport and erosion we now know to be so great ; and in a great submergence and re-elevation of the land, bringing all parts of it and all elevations up to five thousand feet suc- cessively under the influence of these various agencies, along with those of the ocean-currents. They also hold that, at the beginning of the glacial submergence, the land was deeply covered by decomposed .rock, similar to that which still exists on the hills of the Southern States, and which, as Dr. Hunt has shown, would afford not only earthy debris, but large quantities of bowlders ready for transporta- tion by ice.
I would also remark that there has been the greatest possible ex- aggeration as to the erosive action of land-ice. In 1865, after a visit to the Alpine glaciers, I maintained that in these mountains glaciers are relatively protective rather than erosive agencies, and that the detritus which the glacier streams deliver is derived mostly from the atmospherically wasted peaks and cliffs that project above them. Since that time many other observers have maintained like views, and very recently Mr. Davis, of Cambridge, and Mr. A. Irving have ably treated this subject.* Smoothing and striation of rocks are undoubt- edly important effects, both of land-glaciers and heavy sea-borne ice ; but the leveling and filling agency of these is much greater than the erosive. As. a matter of fact, as Newberry, Hunt, Belt, Spencer, and others have shown, the glacial age has dammed up vast numbers of old channels which it has been left for modern streams partially to excavate.
The till, or bowlder clay, has been called a " ground moraine," but there are really no Alpine moraines at all corresponding to it. On the other hand, it is more or less stratified, often rests on soft niateri-
- " Proceedings of the Boston Society of Natural History," xxii ; " Journal of the
Geological Society of London," February, 1883.
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als which glaciers would have swept away, sometimes contains marine shells, or passes into marine clays in its horizontal extension, and in- variably in its imbedded bowlders and its paste shows an unoxidized condition, which could not have existed if it had been a sub-aerial deposit. When the Canadian till is excavated, and exposed to the air, it assumes a brown color, owing to oxidation of its iron ; and many of its stones and bowlders break up and disintegrate under the action of air and frost. These are unequivocal signs of a sub-aqueous deposit. Here and there we find associated with - it, and especially near the bottom and at the top, indications of powerful water-action, as if of land-torrents acting at particular elevations of the land, or heavy surf and ice action on coasts ; and the attempts to explain these by glacial streams have been far from successful. A singular objection sometimes raised against the sub-aqueous origin of the till is its general want of marine remains, but this is by no means universal ; and it is well known that coarse conglomerates of all ages are gener- ally destitute of fossils, except in their pebbles ; and it is further to be observed that the conditions of an ice-laden sea are not those most favorable for the extension of marine life, and that the period of time covered by the glacial age must have been short, compared with that represented by some of the older formations.
This last consideration suggests a question which might afford scope for another address of an hour's duration the question how long time has elapsed since the close of the glacial period. Recently the opinion has been gaining ground that the close of the ice age is very recent. Such reasons as the following lead to this conclusion : The amount of atmospheric decay of rocks and of denudation in gen- eral, which have occurred since the close of the glacial period, are scarcely appreciable ; little erosion of river-valleys or of coast-terraces has occurred. The calculated recession of water-falls and of produc- tion of lake-ridges lead to the same conclusion. So do the recent state of bones and shells in the pleistocene deposits and the perfectly mod- ern facies of their fossils. On such evidence the cessation of the gla- cial cold and settlement of our continents at their present levels are events which may have occurred not more than six thousand or seven thousand years ago, though such time estimates are proverbially un- certain in geology. This subject also carries with it the greatest of all geological problems, next to that of the origin of life ; namely, the origin and early history of man. Such questions can not be discussed in the closing sentences of an hour's address. I shall only draw from them one practical inference. Since the comparatively short post-gla- cial and recent periods apparently include the whole of human history, we are but new-comers on the earth, and therefore have had little opportunity to solve the great problems which it presents to us. But this is not all. Geology as a science scarcely dates from a century ago. We have reason for surprise, in these circumstances, that it has
�� �� � learned so much, but for equal surprise that so many persons appear to think it a complete and full-grown science, and that it is entitled to speak with confidence on all the great mysteries of the earth that have been hidden from the generations before us. Such being the newness of man and of his science of the earth, it is not too much to say that humility, hard work in collecting facts, and abstinence from hasty generalization, should characterize geologists, at least for a few generations to come.
In conclusion, science is light, and light is good; but it must be carried high, else it will fail to enlighten the world. Let us strive to raise it high enough to shine over every obstruction which casts any shadow on the true interests of humanity. Above all, let us hold up the light, and not stand in it ourselves.
- Address of the President of the American Association for the Advancement of Science, delivered at Minneapolis, August 15, 1883. Reprinted from "Science."