Geological Evidences of the Antiquity of Man/Chapter 18
CHAPTER XVIII.
THE GLACIAL PERIOD IN NORTH AMERICA.
POST-GLACIAL STRATA CONTAINING REMAINS OF MASTODON GIGANTEUS IN NORTH AMERICA—SCARCITY OF MARINE SHELLS IN GLACIAL DRIFT OF CANADA AND THE UNITED STATES—GREATER SOUTHERN EXTENSION OF ICE-ACTION IN NORTH AMERICA THAN IN EUROPE—TRAINS OF ERRATIC BLOCKS OF VAST SIZE IN BERKSHIRE, MASSACHUSETTS—DESCRIPTION OF THEIR LINEAR ARRANGEMENT AND POINTS OF DEPARTURE—THEIR TRANSPORTATION REFERRED TO FLOATING AND COAST ICE—GENERAL REMARKS ON THE CAUSES OF FORMER CHANGES OF CLIMATE AT SUCCESSIVE GEOLOGICAL EPOCHS—SUPPOSED EFFECTS OF THE DIVERSION OF THE GULF STREAM IN A NORTHERLY INSTEAD OF NORTH-EASTERLY DIRECTION—DEVELOPMENT OF EXTREME COLD ON THE OPPOSITE SIDES OF THE ATLANTIC IN THE GLACIAL PERIOD NOT STRICTLY SIMULTANEOUS—NUMBER OF SPECIES OF PLANTS AND ANIMALS COMMON TO PRE-GLACIAL AND POST-GLACIAL TIMES.
ON the North American Continent, between the arctic circle and the 42nd parallel of latitude, we meet with signs of ice-action on a scale as grand if not grander than in Europe; and there also the excess of cold appears to have been first felt, at the close of the tertiary, and to have continued throughout a large portion of the post-pliocene period.
The general absence of organic remains in the North American glacial formation, makes it as difficult as in Europe, to determine what mammalia lived on the continent at the time of the most intense refrigeration, or when extensive areas were becoming strewed over with glacial drift and erratic blocks, but it is certain that a large proboscidean now extinct, the Mastodon giganteus Cuv., together with many other quadrupeds, some of them now living and others extinct, played a conspicuous part in the post-glacial era. By its frequency as a fossil species, this pachyderm represents the European Elephas primigenius, although the latter also occurs fossil in the United States and Canada, and abounds, as I learn from Sir John Richardson, in latitudes farther north than those to which the mastodon has been traced.
In the state of New York, the mastodon is not unfrequently met with in bogs and lacustrine deposits formed in hollows in the drift, and therefore, in a geological position, much resembling that of recent peat and shell-marl in the British Isles, Denmark, or the Valley of the Somme, as before described. Sometimes entire skeletons have been discovered within a few feet of the surface, in peaty earth at the bottom of small ponds, which the agriculturists had drained. The shells in these cases belong to freshwater genera, such as Limnea, Physa, Planorbis, Cyclas, and others, differing from European species, but the same as those now proper to ponds and lakes in the same parts of America.
I have elsewhere given an account of several of these localities which I visited in 1842,[1] and can state that they certainly have a more modern aspect than almost all the European deposits in which remains of the mammoth occur, although a few instances are cited of Elephas primigenius having been dug out of peat in Great Britain. Thus I was shown a mammoth's tooth in the museum at Torquay, in Devonshire, which is believed to have been dredged up from a deposit of vegetable matter now partially submerged beneath the sea. A more elevated part of the same peaty formation constitutes the bottom of the valley in which Tor Abbey stands. This individual elephant must certainly have been of more modern date than his fellows found fossil in the gravel of the Brixham cave, before described (p. 100), for it flourished when the physical geography of Devonshire, unlike that of the cave period, was almost identical with that now established.
I cannot help suspecting that many tusks and teeth of the mammoth, said to have been found in peat, may be as spurious as are the horns of the rhinoceros cited more than once in the 'Memoirs of the Wernerian Society,' as having been obtained from shell-marl in Forfarshire and other Scotch counties; yet, between the period when the mammoth was most abundant, and that when it died out, there must have elapsed a long interval of ages when it was growing more and more scarce; and we may expect to find occasional stragglers buried in deposits long subsequent in date to others, until at last we may succeed in tracing a passage from the post-pliocene to the recent fauna, by geological monuments, which will fill up the gap before alluded to (p. 144) as separating the era of the flint tools of Amiens and Abbeville from that of the peat of the Valley of the Somme.
How far the lacustrine strata of North America, above mentioned, may help to lessen this hiatus, and whether some individuals of the Mastodon giganteus may have come down to the confines of the historical period, is a question not so easily answered as might at first sight be supposed. A geologist might naturally imagine that the fluviatile formation of Goat Island, seen at the falls of Niagara, and at several points below the falls,[2] was very modern, seeing that the fossil shells contained in it are all of species now inhabiting the waters of the Niagara, and seeing also that the deposit is more modern than the glacial drift of the same locality. In fact, the old river bed, in which bones of the mastodon occur, holds the same position relatively to the boulder formation as the strata of shell-marl and boggy-earth, with bones of mastodon, so frequent in the State of New York, bear to the glacial drift, and all may be of contemporaneous date. But in the case of the valley of the Niagara, we happen to have a measure of time, which is wanting in the other localities, namely, the test afforded by the recession of the falls, an operation still in progress, by which the deep ravine of the Niagara, seven miles long, between Queenstown and Goat Island, has been hollowed out. This ravine is not only post-glacial, but also posterior in date to the fluviatile or mastodon-bearing beds. The individual therefore found fossil near Goat Island flourished before the gradual excavation of the deep and long chasm, and we must reckon its antiquity, not by thousands, but by tens of thousands of years, if I have correctly estimated the minimum of time which was required for the erosion of that great ravine.[3]
The stories widely circulated of bones of the mastodon having been observed with their surfaces pierced as if by arrow-heads, or bearing the marks of wounds inflicted by some stone implement, must in future be more carefully inquired into, for we can scarcely doubt that the mastodon in North America lived down to a period when the mammoth coexisted with man in Europe. But I need say no more on this subject, having already (p. 200) explained my views in regard to the evidence of the antiquity of man in North America, when treating of the human bone discovered at Natchez, on the Mississippi.
In Canada and the United States, we experience the same difficulty as in Europe, when we attempt to distinguish between glacial formations of submarine and those of supramarine origin. In the New World, as in Scotland and England, marine shells of this era have rarely been traced higher than five hundred feet above the sea, and seven hundred feet seems to be the maximum to which at present they are known to ascend. In the same countries, erratic blocks have travelled from N. to S., following the same direction as the glacial furrows and striæ imprinted almost everywhere on the solid rocks underlying the drift. Their direction rarely deviates more than fifteen degrees E. or W. of the meridian, so that we can scarcely doubt, in spite of the general dearth of marine shells, that icebergs floating in the sea, and often running aground on its rocky bottom, were the instruments by which most of the blocks were conveyed to southern latitudes.
There are, nevertheless, in the United States, as in Europe, several groups of mountains which have acted as independent centres for the dispersion of erratics, as, for example, the White Mountains, latitude 44° N., the highest of which, Mount Washington, rises to about 6,300 feet above the sea; and according to Professor Hitchcock, some of the loftiest of the hills of Massachusetts once sent down their glaciers into the surrounding lower country.
Great southern Extension of Trains of Erratic Blocks in Berkshire, Massachusetts, U.S., lat. 42° N.
Having treated so fully in this volume of the events of the glacial period, I am unwilling to conclude without laying before the reader the evidence displayed in North America, of ice-action in latitudes farther south, by about ten degrees than any seen on an equal scale in Europe. This extension southwards of glacial phenomena, in regions where there are no snow-covered mountains like the Alps to explain the exception, nor any hills of more than moderate elevation, constitutes a feature of the western as compared to the eastern side of the Atlantic, and must be taken into account when we speculate on the causes of the refrigeration of the northern hemisphere during the post-pliocene period.
In 1852, accompanied by Mr. James Hall, State geologist of New York, author of many able and well-known works on geology and paleontology, I examined the glacial drift and erratics of the county of Berkshire, Massachusetts, and those of the adjoining parts of the State of New York, a district about 130 miles inland from the Atlantic coast, and situated due west of Boston, in lat. 42° 25′ north. This latitude corresponds in Europe to that of the north of Portugal. Here numerous detached fragments of rock are seen, having a linear arrangement or being continuous in long parallel trains, running nearly in straight lines over hill and dale for distances of five, ten, and twenty miles, and sometimes greater distances. Seven of the more conspicuous of these trains, from 1 to 7 inclusive, fig. 50, are laid down in the accompanying map or ground plan.[4] It will be remarked that they run in a NW. and SE. direction, or almost transversely to the ranges of hills a, b, and c, which run NNE. and SSW. The crests of these chains are about 800 feet in height above the intervening valleys. The blocks of the northernmost train, No. 7, are of limestone, derived from the calcareous chain b; those of the two trains next to the south, Nos. 6 and 5, are composed exclusively in the first part of their course of a green chloritic rock of great toughness, but after they have passed the ridge b, a mixture of calcareous blocks is observed. After traversing the valley for a distance of six miles, these two trains pass through depressions or gaps in the range c, as they had previously done in crossing the range b, showing that the dispersion of the erratics bears some relation to the actual inequalities of the surface, although the
course of the same blocks is perfectly independent of the more leading features of the geography of the country, or those by which the present lines of drainage are determined. The greater number of the green chloritic fragments in Fig. 50
Distance in a straight line, between the mountain ranges a and c, about eight miles.
a Canaan range, in the State of New York. The crest consists of green chloritic rock.
b Richmond range, the western division of which consists in Merriman's Mount of the same green rock as a, but in a more schistose form, while the eastern division is composed of slaty limestone.
c The Lenox range, consisting in part of mica-schist, and in some districts of crystalline limestone.
d Knob in the range a, from which most of the train No. 6 is supposed to have been derived.
e Supposed starting point of the train No. 5 in the range a.
f Hiatus of 175 yards, or space without blocks.
g Sherman's House.
h Perry's Peak.
k Flat Rock.
l Merriman's Mount.
- m Dupey's Mount.
- n Largest block of train, No. 6. See figs. 51 and 52, p. 359.
- p Point of divergence of part of the train No. 6, where a branch is sent off to No. 5.
- No. 1 The most southerly train examined by Messrs. Hall and Lyell, between Stockbridge and Richmond, composed of blocks of black slate, blue limestone, and some of the green Canaan rock, with here and there a boulder of white quartz.
- No. 2 Train composed chiefly of large limestone masses, some of them divided into two or more fragments, by natural joints.
- No. 3 Train composed of blocks of limestone and the green Canaan rock; passes south of the Richmond Station on the Albany and Boston railway; is less defined than Nos. 1 and 2.
- No. 4 Train chiefly of limestone blocks, some of them thirty feet in diameter, running to the north-west of the Richmond Station, and passing south of the Methodist Meeting-house, where it is intersected by a railway cutting.
- No. 5 South train of Dr. Reid, composed entirely of large blocks of the green chloritic Canaan rock; passes north of the Old Richmond Meeting-house, and is three-quarters of a mile north of the preceding train (No. 4).
- No. 6 The great or principal train (north train of Dr. Reid), composed of very large blocks of the Canaan rock, diverges at p, and unites by a branch with train No. 5.
- No. 7 A well-defined train of limestone blocks, with a few of the Canaan rock, traced from the Richmond to the slope of the Lenox range.
trains 5 and 6 have evidently come from the ridge a, and a large proportion of the whole from its highest summit, d, where the crest of the ridge has been worn into those dome-shaped masses called 'roches moutonnées,' already alluded to (pp. 269 and 293), and where several fragments having this shape, some of them thirty feet long, are seen in situ, others only slightly removed from their original position, as if they had been just ready to set out on their travels. Although smooth and rounded on their tops, they are angular on their lower parts, where their outline has been derived from the natural joints of the rock. Had these blocks been conveyed from d by glaciers, they would have radiated in all directions from a centre, whereas not one even of the smaller ones is found to the westward of a, though a very slight force would have made them roll down to the base of that ridge, which is very steep on its western declivity. It is clear, therefore, that the propelling power, whatever it may have been, acted exclusively in a south-easterly direction. Professor Hall and I observed one of the green blocks, twenty-four feet long, poised upon another about nineteen feet in length. The largest of all on the west flank of m, or Dupey's Mount, called the Alderman, is above ninety feet in diameter,
Fig. 51
Fig. 52
Section showing position of the block, fig. 51.
a The large block. Fig. 51 and n map, p. 357.
b Fragment detached from the same.
c Unstratified drift with boulders.
d Silurian limestone in inclined stratification.
and nearly three hundred feet in circumference. We counted at some points between forty and fifty blocks visible at once, the smallest of them larger than a camel.
The annexed drawing represents one of the best known of train No. 6, being that marked n on the map, p. 357. According to our measurement it is fifty-two feet long by forty in width, its height above the drift in which it is partially buried being fifteen feet. At the distance of several yards occurs a smaller block, three or four feet in height, twenty feet long, and fourteen broad, composed of the same compact chloritic rock, and evidently a detached fragment from the bigger mass, to the lower and angular part of which it would fit on exactly. This erratic n has a regularly rounded top, worn and smoothed like the roches moutonnées before mentioned, but no part of the attrition can have occurred since it left its parent rock, the angles of the lower portion being quite sharp and unblunted.
From railway cuttings through the drift of the neighbourhood, and other artificial excavations, we may infer that the position of the block n, if seen in a vertical section, would be as represented in fig. 52. The deposit c in that section, p. 359, consists of sand, mud, gravel, and stones, for the most part unstratified, resembling the till or boulder clay of Europe. It varies in thickness from ten to fifty feet, being of greater depth in the valleys. The uppermost portion is occasionally, though rarely, stratified. Some few of the imbedded stones have flattened, polished, striated, and furrowed sides. They consist invariably, like the seven trains above mentioned, of kinds of rock confined to the region lying to the NW., none of them having come from any other quarter. Whenever the surface of the underlying rock has been exposed by the removal of the superficial detritus, a polished and furrowed surface is seen, like that underneath a glacier, the direction of the furrows being from NW. to SE., or corresponding to the course of the large erratics.
As all the blocks, instead of being dispersed from a centre, have been carried in one direction, and across the ridges a, b, c, and the intervening valleys, the hypothesis of glaciers is out of the question. I conceive, therefore, that the erratics were conveyed to the places they now occupy by coast ice, when the country was submerged beneath the waters of a sea cooled by icebergs coming annually from arctic regions.
Fig. 53
d, e Masses of floating ice carrying fragments of rock.
Suppose the highest peaks of the ridges a, b, c, in the annexed diagram, to be alone above water, forming islands, and d e to be masses of floating ice, which drifted across the Canaan and Richmond valleys at a time when they were marine channels, separating islands, or rather chains of islands, having a NNE. and SSW. direction. A fragment of ice such as d, freighted with a block from a, might run aground, and add to the heap of erratics at the NW. base of the island (now ridge) b, or, passing through a sound between b and the next island of the same group, might float on till it reached the channel between b and c. Year after year two such exposed cliffs in the Canaan range as d and e of the map, fig. 50, p. 357, undermined by the waves, might serve as the points of departure of blocks, composing the trains Nos. 5 and 6. It may be objected that oceanic currents could not always have had the same direction; this may be true, but during a short season of the year when the ice was breaking up the prevailing current may have always run SE.
If it be asked why the blocks of each train are not more scattered, especially when far from their source, it may be observed, that after passing through sounds separating islands, they issued again from a new and narrow starting point; moreover, we must not exaggerate the regularity of the trains, as their width is sometimes twice as great in one place as in another; and No. 6 sends off a branch at p, which joins No. 5. There are also stragglers, or large blocks, here and there in the spaces between the two trains. As to the distance to which any given block would be carried, that must have depended on a variety of circumstances; such as the strength of the current, the direction of the wind, the weight of the block, or the quantity and draught of the ice attached to it. The smaller fragments would, on the whole, have the best chance of going farthest; because, in the first place, they were more numerous, and then, being lighter, they required less ice to float them, and would not ground so readily on shoals, or, if stranded, would be more easily started again on their travels. Many of the blocks, which at first sight seem to consist of single masses, are found, when examined, to be made up of two, three, or more pieces, divided by natural joints. In case of a second removal by ice, one or more portions would become detached and be drifted to different points further on. Whenever this happened, the original size would be lessened, and the angularity of the block previously worn by the breakers would be restored, and this tendency to split may explain why some of the far-transported fragments remain very angular.
These various considerations may also account for the fact that the average size of the blocks of all the seven trains laid down on the plan, fig. 50, lessens sensibly in proportion as we recede from the principal points of departure of particular kinds of erratics, yet not with any regularity, a huge block now and then recurring when the rest of the train consists of smaller ones.
All geologists acquainted with the district now under consideration are agreed that the mountain ranges a, b, and c, as well as the adjoining valleys, had assumed their actual form and position before the drift and erratics accumulated on and in them, and before the surface of the fixed rocks was polished and furrowed. I have the less hesitation in ascribing the transporting power to coast-ice, because I saw, in 1852, an angular block of sandstone, eight feet in diameter, which had been brought down several miles by ice, only three years before, to the mouth of the Petitcodiac estuary, in Nova Scotia, where it joins the Bay of Fundy; and I ascertained that on the shores of the same bay, at the South Joggins, in the year 1850, much larger blocks had been removed by coast-ice, and after they had floated half a mile, had been dropped in salt water by the side of a pier built for loading vessels with coal, so that it was necessary at low tide to blast these huge ice-borne rocks with gunpowder, in order that the vessels might be able to draw up alongside the pier. These recent exemplifications of the vast carrying powers of ice occurred in lat. 46° N. (corresponding to that of Bordeaux), in a bay never invaded by icebergs.
I may here remark that a sheet of ice of moderate thickness, if it extend over a wide area, may suffice to buoy up the largest erratics which fall upon it. The size of these will depend, not on the intensity of the cold, but on the manner in which the rock is jointed, and the consequent dimensions of the blocks into which it splits, when falling from an undermined cliff.
When I first endeavoured in the 'Principles of Geology,' in 1830,[5] to explain the causes, both of the warmer and colder climates, which have at former periods prevailed on the globe, I referred to successive variations in the height and position of the land, and its extent relatively to the sea in polar and equatorial latitudes—also to fluctuations in the course of oceanic currents and other geographical conditions, by the united influence of which I still believe the principal revolutions in the meteorological state of the atmosphere at different geological periods have been brought about. The Gulf Stream was particularly alluded to by me as moderating the winter climate of northern Europe, and as depending for its direction on temporary and accidental peculiarities, in the shape of the land, especially that of the narrow Straits of Bahama, which a slight modification in the earth's crust would entirely alter.
Mr. Hopkins, in a valuable essay on the causes of former changes of climate,[6] has attempted to calculate how much the annual temperature of Europe would be lowered if this Gulf Stream were turned in some other and new direction, and estimates the amount at about six or seven degrees of Fahrenheit. He also supposes that if at the same time a considerable part of northern and central Europe were submerged, so that a cold current from the arctic seas should sweep over it, an additional refrigeration of three or four degrees would be produced. He has speculated in the same essay on the effects which would be experienced in the eastern hemisphere if the same mighty current of warm water, instead of crossing the Atlantic, were made to run northwards from the Gulf of Mexico through the region now occupied by the valley of the Mississippi, and so onwards to the arctic regions.
After reflecting on what has been said in the thirteenth chapter of the submergence and re-elevation of the British Isles and the adjoining parts of Europe, and the rising and sinking of the Alps, and the basins of some of the great rivers flowing from that chain, since the commencement of the glacial period, a geologist will not be disposed to object to the theory above adverted to, on the score of its demanding too much conversion of land into sea, or almost any amount of geographical change in post-pliocene times. But a difficulty of another kind presents itself. We have seen that, during the glacial period, the cold in Europe extended much farther south than it does at present, and in this chapter we have demonstrated that in North America the cold also extended no less than 10° of latitude still farther southwards than in Europe; so that if a great body of heated water, instead of flowing north-eastward, were made to pass through what is now the centre of the American continent towards the Arctic circle, it could not fail to mitigate the severity of the winter's cold in precisely those latitudes where the cold was greatest, and where it has left monuments of ice-action surpassing in extent any exhibited on the European side of the ocean.
In the actual state of the globe, the isothermal lines, or rather the lines of equal winter temperature, when traced eastward from Europe to North America, bend 10° south, there being a marked excess of winter cold in corresponding latitudes west of the Atlantic. During the glacial period, viewing it as a whole, we behold signs of a precisely similar deflection of these same isochimenal lines when followed from east to west; so that if, in the hope of accounting for the former severity of glacial action in Europe, we suppose the absence of the Gulf Stream and imagine a current of equivalent magnitude to have flowed due north from the Gulf of Mexico, we introduce, as we have just hinted, a source of heat into precisely that part of the continent where the extreme conditions of refrigeration are most manifest. Viewed in this light, the hypothesis in question would render the glacial phenomena described in the present chapter more perplexing and anomalous than ever. But here another question arises, whether the eras at which the maximum of cold was attained on the opposite sides of the Atlantic were really contemporaneous? We have now discovered not only that the glacial period was of vast duration, but that it passed through various phases and oscillations of temperature; so that, although the chief polishing and furrowing of the rocks and transportation of erratics in Europe and North America may have taken place contemporaneously, according to the ordinary language of geology, or when the same testacea and the same post-pliocene assemblage of mammalia flourished, yet the extreme development of cold on the opposite sides of the ocean may not have been strictly simultaneous, but, on the contrary, the one may have preceded or followed the other by a thousand or more than a thousand centuries.
It is probable that the greatest refrigeration of Norway, Sweden, Scotland, Wales, the Vosges, and the Alps coincided very nearly in time; but when the Scandinavian and Scotch mountains were encrusted with a general covering of ice, similar to that now enveloping Greenland, this last country may not have been in nearly so glacial a condition as now, just as we find that the old icy crust and great glaciers, which have left their mark on the mountains of Norway and Sweden, have now disappeared, precisely at a time when the accumulation of ice in Greenland is so excessive. In other words, we see that in the present state of the northern hemisphere, at the distance of about fifteen hundred miles, two meridional zones, enjoying very different conditions of temperature, may co-exist, and we are, therefore, at liberty to imagine some former alternations of colder and milder climates on the opposite sides of the ocean throughout the post-pliocene era of a compensating kind, the cold on the one side balancing the milder temperature on the other. By assuming such a succession of events we can more easily explain why there has not been a greater extermination of species, both terrestrial and aquatic, in polar and temperate regions, during the glacial epoch, and why so many species are common to pre-glacial and post-glacial times.
The numerous plants which are common to the temperate zones N. and S. of the equator have been referred by Mr. Darwin and Dr. Hooker to migrations, which took place along mountain chains running from N. to S. during some of the colder phases of the glacial epoch.[7] Such an hypothesis enables us to dispense with the doctrine that the same species ever originated independently in two distinct and distant are as; and it becomes more feasible if we admit the doctrine of the co-existence of meridional belts of warmer and colder climate, instead of the simultaneous prevalence of extreme cold both in the eastern and western hemisphere. It also seems necessary, as colder currents of water always flow to lower latitudes, while warmer ones are running towards polar regions, that some such compensation should take place, and that an increase of cold in one region must to a certain extent be balanced by a mitigation of temperature elsewhere.
Sir John F. Herschel, in his recent work on 'Physical Geography,' when speaking of the open sea which is caused in part of the polar regions by the escape of ice through Behring's Straits, and the flow of warmer water northwards through the same channel, observes that these straits, by which the continents of Asia and North America are now parted, 'are only thirty miles broad where narrowest, and only twenty-five fathoms in their greatest depth.' But 'this narrow channel,' he adds, 'is yet important in the economy of nature, inasmuch as it allows a portion of the circulating water from a warmer region to find its way into the polar basin, aiding thereby not only to mitigate the extreme rigour of the polar cold, but to prevent in all probability a continual accretion of ice, which else might rise to a mountainous height.'[8]
Behring's Straits, here alluded to, happen to agree singularly in width and depth with the Straits of Dover, the difference in depth not being more than three or four feet; so that at the rate of upheaval, which is now going on in many parts of Scandinavia, of two and a half feet in a century, such straits might be closed in 3000 years, and a vast accumulation of ice to the northward commence forthwith.
But, on the other hand, although such an accumulation might spread its refrigerating influence for many miles southwards beyond the new barrier, the warm current which now penetrates through the straits, and which at other times is chilled by floating ice issuing from them, would, when totally excluded from all communication with the icy sea, have its temperature raised and its course altered, so that the climate of some other area must immediately begin to improve.
The scope and limits of this volume forbid my pursuing these speculations and reasonings farther; but I trust I have said enough to show that the monuments of the glacial period, when more thoroughly investigated, will do much towards expanding our views as to the antiquity of the fauna and flora now contemporary with man, and will therefore enable us the better to determine the time at which man began in the northern hemisphere to form part of the existing fauna.
- ↑ Travels in North America, vol. i. p. 55, London, 1845; and Manual of Geology, ch. xii. 5th ed. p. 144.
- ↑ Travels in North America, by the Author, vol. i. ch. ii.; and vol. ii. ch. xix.
- ↑ Principles of Geology, 9th ed. p. 2; and Travels in North America, vol. i. p. 32, 1845.
- ↑ This ground plan, and a further account of the Berkshire erratics, was given in an abstract of a lecture delivered by me to the Royal Institution of Great Britain, April 27, 1855, and published in their Proceedings.
- ↑ 1st edit. ch. vii.; 9th edit. ib.
- ↑ Hopkins, Geological Quarterly Journal, vol. viii. p. 56, 1852.
- ↑ Darwin, Origin of Species, ch. xi. p. 365; Hooker, Flora of Australia, Introduction, p. 18.
- ↑ Herschel's Physical Geography, p. 41, 1861.