Geology and Mineralogy considered with reference to Natural Theology/Chapter 8
Strata of the Secondary Series.
We may consider the history of secondary, and also of tertiary strata, in two points of view: the one, respecting their actual state as dry land, destined to be the habitation of man; the other, regarding their prior condition, whilst in progress of formation at the bottom of the waters, and occupied by crowds of organic beings in the enjoyment of life.
With regard to their adaptation to human uses, it may be stated generally, that the greater number of the most populous and highly civilized assemblages of mankind inhabit those portions of the earth which are composed of secondary and tertiary formations. Viewed, therefore, in their relations to that agricultural stage of human society in which man becomes established in a settled habitation, and applies his industry to till the earth, we find in these formations which have been accumulated in apparently accidental succession, an arrangement highly advantageous to the cultivation of their surface. The movements of the waters, by which the materials of strata have been transported to their present place, have caused them to be intermixed in such manner, and in such proportions, as are in various degrees favourable to the growth of the different vegetable productions, which man requires for himself and the domestic animals he has collected around him.
The process is obvious whereby even solid rocks are converted into soil fit for the maintenance of vegetation, by simple exposure to atmospheric agency; the disintegration produced by the vicissitudes of heat and cold, moisture and dryness, reduces the surface of almost all strata to a comminuted state of soil, or mould, the fertility of which is usually in proportion to the compound nature of its ingredients.
The three principal materials of all strata are the earths of flint, clay, and lime; each of these, taken singly and in a state of purity, is comparatively barren: the admixture of a small proportion of clay gives tenacity and fertility to sand, and the further addition of calarious earth produces a soil highly valuable to the agriculturist: and where the natural proportions are not adjusted in the most beneficial manner, the facilities afforded by the frequent juxta-position of lime, or marl, or gypsum, for the artificial improvement of those soils which are defective in these ingredients, add materially to the earth's capability of adaptation to the important office of producing food. Hence it happens that the great corn-fields, and the greatest population of the world, are placed on strata of the secondary and tertiary formations; or on their detritus, composing still more compound, and consequently more fertile diluvial, and alluvial deposites.
Another advantage in the disposition of stratified rocks consists in the fact that strata of limestone, sand, and sandstone which readily absorb water, alternate with beds of clay, or marl, which are impermeable to this most important fluid. All permeable strata receive rain-water at their surface, whence it descends until it is arrested by an impermeable subjacent bed of clay, causing it to accumulate throughout the lower region of each porous stratum, and to form extensive reservoirs, the overflowings of which on the sides of valleys constitute the ordinary supply of springs and rivers. These reservoirs are not only occasional crevices and caverns, but the entire space of all the small interstices of those lower parts of each permeable stratum, which are beneath the level of the nearest flowing springs. Hence if a well be sunk to the water-bearing level of any stratum, it forms a communication with a permanent subterranean sheet of water, affording plentiful supplies to the inhabitants of upland districts, which are above the level of natural springs.
A further benefit which man derives from the disposition of the mineral ingredients of the secondary strata, results from the extensive diffusion of muriate of soda, or common salt, throughout certain portions of these strata, especially those of the new red sandstone formation. Had not the beneficent providence of the Creator laid up these stores of salt within the bowels of the earth, the distance of inland countries from the sea would have rendered this article of prime and daily necessity, unattainable to a large proportion of mankind: but, under the existing dispensation, the presence of mineral salt, in strata which are dispersed generally over the interior of our continents and larger islands, is a source of health, and daily enjoyment, to the inhabitants of almost every region of the earth. Muriate of soda is also among the most abundant of the saline compounds formed by sublimation in the craters of volcanos.
With respect to the state of animal life, during the deposition of the Secondary strata, although the petrified re. mains of Zoophytes, Crustacea, Testacea, and Fishes, show that the seas in which these strata were formed, like those which gave birth to the Transition series, abounded with creatures referable to the four existing divisions of the animal kingdom, still the condition of the globe seems not yet to have been sufficiently advanced in tranquillity, to admit of general occupation by warm-blooded terrestrial Mammalia.
The only terrestrial Mammalia yet discovered in any secondary stratum, are the small marsupial quadrupeds allied to the Opossum, which occur in the oolite formation, at Stonesfield, near Oxford. The jaws of two species of this genus are represented in Plate 2. A. B; the double roots of the molar teeth at once refer these jaws to the class of Mammalia, and the form of their crowns places them in the order of Marsupial animals. Two other small species have been discovered by Cuvier, in the tertiary formations of the basin of Paris, in the gypsum of Mont Martre.
The Marsupial Order comprehends a large number of existing genera, both herbivorous and carnivorous, which are now peculiar to North and South America, and to New Holland, with the adjacent islands. The kangaroo and opossum are its most familiar examples. The name of Marsupialia is derived from the presence of a large external marsupium, or pouch, fixed on the abdomen, in which the fœtus is placed after a very short period of uterine gestation, and remains suspended to the nipple by its mouth, until sufficiently matured to come forth to the external air. The discovery of animals of this kind, both in the secondary and tertiary formations, shows that the Marsupial Order, so far from being of more recent introduction than other orders of mammalia, is in reality the first and most ancient condition, under which animals of this class appeared upon our planet: as far as we know, it was their only form during the secondary period; it was co-exist ant with many other orders in the early parts of the tertiary period; and its geographical distribution in the present creation, is limit; ed to the regions we have above enumerated.
The peculiar feature in the population of the whole series of secondary strata, was the prevalence of numerous and gigantic forms of Saurian reptiles; Many of these were exclusively marine; others amphibious: others were terrestrial, ranging in savannas and jungles, clothed with a tropical vegetation, or basking on the margins of estuaries, lakes, and rivers. Even the air was tenanted by flying lizards, under the dragon form of Pterodactyles. The earth was probably at that time too much covered with water, and those portions of land which had emerged above the surface, were too frequently agitated by earthquakes, inundations, and atmospheric irregularities, to be extensively occupied by any higher order of quadrupeds than reptiles.
As the history of these reptiles, and also that of the vegetable remains, of the secondary formations, will be made a subject of distinct inquiry, it will here suffice to state, that the proofs of method and design in the adaptation of these extinct forms of organization to the varied circumstances and conditions of the earth's progressive stages of advancement, are similar to those we trace in the structure of living animal and vegetable bodies; in each case we argue that the existence of contrivances, adapted to produce definite and useful ends, implies the anterior existence and agency of creative intelligence.
- The secondary strata are composed of extensive beds of sand and sandstone, mixed occasionally with pebbles, and alternating with deposites of clay, and marl, and limestone. The materials of most of these strata appear to have been derived from the detritus of primary and transition rocks; and the larger fragments, which are preserved in the form of pebbles, often indicate the sources from which these rounded fragments were supplied.
The transport of these materials from the site of older formations to their place in the secondary series, and their disposition in strata widely extended over the bottom of the early seas, seem to have resulted from forces, producing the destruction of more ancient lands, on a scale of magnitude unexampled among the actual phenomena of moving waters.
- It is no small proof of design in the arrangement of the materials that compose the surface of our earth, that whereas the primitive and granitic rocks are least calculated to afford a fertile soil, they are for the most part mule to constitute the mountain districts of the world, which, from, their elevation and irregularities, would otherwise be but ill adapted for human habitation; while the lower and more temperate regions are, usually composed of derivative, or secondary strata, in which the compound nature of their ingredients qualifies them to be of the greatest utility to mankind, by their subserviency to the purposes of luxuriant vegetation.—Buckland's Inaugural Lecture, oxford, 1820, p. 17.
- Although the most frequent position of rock salt, and of salt springs, is in strata of the new red sandstone formation, which has consequently been designated by some geologists as the saliferous system, yet it is not exclusively confined to them. The salt mines of Wieliezka and Sicily are in tertiary formations; those of Cardona in cretaceous; some of those in the Tyrol in the oolites; and near Durham there are salt springs in the coal formation.
- In a highly important physiological paper, in the Phil. Trans. London, 1834, part ii. p. 349, Mr. Owen has pointed out "the most irrefragible evidence of creative foresight, afforded by the existing Marsupialia, in the peculiar modifications both of the maternal and fœtal system, designed with especial reference to each other's peculiar condition." With respect to the final cause of these peculiarities, he conjectures that they have relation to an inferior condition of the brain and nervous system in the Marsupialia; and considers the more protracted period of viviparous utero-gestation in the higher orders of Mammalia to be connected with their fuller development of the parts subservient to the sensorial functions; the more simple form and inferior condition of the brain in Marsupialia, being attended with a lower degree of intelligence, and less perfect condition of the organs of voice.
As this inferior condition of living Marsupialia shows this order to hold an intermediate place between viviparous and oviparous animals, forming, as it were, a link between Mammalia and Reptiles; the analogies afforded by the occurrence of the more simple forms of other classes of animals in the earlier geological deposites, would lead us to expect also that the first forms of Mammalia would have been Marsupial.
In a recent letter to myself; Mr. Owen adds the following interesting particulars respecting the physiology of this remarkable class of animals. "Of the generality of the law, as regards the simple unconvoluted form of the cerebrum in the Marsupials, I have had additional confirmation from recent dissections of a Dasyurus and Phalangista. With an organization defective in that part which I believe to be essential to the docility of the horse, and sagacity of the dog, it is natural to suppose that the Marsupial series of warm-blooded quadrupeds would be insufficient for the great purposes of the Creator, when the earth was rendered fit for the habitation of man. They do, indeed, afford the wandering savages of Australia a partial supply of food; but it is more than doubtful that any of the species will be preserved by civilized man on the score of utility. The more valuable and tractable ruminants are already fast encroaching on the plains where the kangaroo was once the sole representative of the graminivorous Mammalia.
"It is interesting, however, to observe, that the Marsupials, including the Monotremes, form a very complete series, adapted to the assimilation of every form of organic matter; and, no doubt, with enough of instinctive precaution, to preserve themselves from extermination, when surrounded with enemies of no higher intellectual powers than the Reptilia. It would, indeed, be a strong support to the consideration of them as a distinct ovoviviparous sub-class of Mammals, if they should be found as hitherto, to be the sole representatives of the highest class of Vertebrata, in the secondary strata."—R. Owen.
- The vegetable remains of the secondary strata differ from those of the transition period, and are very rarely accumulated into beds of valuable coal. The imperfect coal of the Cleveland Moorlands near Whitby, on the, coast of Yorkshire, and that of Brora in the county of Sutherland, occurs in the lower region of the oolite formation; that of Bückeberg in Nassau, is in the lower region of the same formation, and is of superior quality.