Part 22 (1/2)
We have already endeavoured to establish the origin of lime, in speaking of the Silurian and Devonian periods, but it may be useful to recapitulate the explanation here, even at the risk of repeating ourselves.
We have said that lime was, in all probability, introduced to the globe by thermal waters flowing abundantly through the fissures, dislocations, and fractures in the ground, which were themselves caused by the gradual cooling of the globe; the central nucleus being the grand reservoir and source of the materials which form the solid crust. In the same manner, therefore, as the several eruptive substances--such as granites, porphyries, trachytes, basalts, and lava--have been ejected, so have thermal waters charged with carbonate of lime, and often accompanied by silica, found their way to the surface in great abundance, through the fissures, fractures, and dislocations in the crust of the earth. We need only mention here the Iceland geysers, the springs of Plombieres, and the well-known thermal springs of Bath and elsewhere in this country.
But how comes lime in a state of bicarbonate, dissolved in these thermal waters, to form rocks? That is what we propose to explain.
During the primary geological periods, thermal waters, as they reached the surface, were discharged into the sea and united themselves with the waves of the vast primordial ocean, and the waters of the sea became sensibly calcareous--they contained, it is believed, from one to two per cent. of lime. The innumerable animals, especially Zoophytes, and Mollusca with solid sh.e.l.ls, with which the ancient seas swarmed, secreted this lime, out of which they built up their mineral dwelling--or sh.e.l.l. In this liquid and chemically calcareous medium, the Foraminifera and Polyps of all forms swarmed, forming an innumerable population. Now what became of the bodies of these creatures after death? They were of all sizes, but chiefly microscopic; that is, so small as to be individually all but invisible to the naked eye. The perishable animal matter disappeared in the bosom of the waters by decomposition, but there still remained behind the indestructible inorganic matter, that is to say, the carbonate of lime forming their testaceous covering; these calcareous deposits acc.u.mulating in thick beds at the bottom of the sea, became compacted into a solid ma.s.s, and formed a series of continuous beds superimposed on each other. These, increasing imperceptibly in the course of ages, ultimately formed the rocks of the _Cretaceous_ period, which we have now under consideration.
These statements are not, as the reader might conceive from their nature, a romantic conception invented to please the imagination of those in search of a system--the time is past when geology should be regarded as the romance of Nature--nor has what we advance at all the character of an arbitrary conception. One is no doubt struck with surprise on learning, for the first time, that all the limestone rocks, all the calcareous stones employed in the construction of our dwellings, our cities, our castles and cathedrals, were deposited in the seas of an earlier world, and are only composed of an aggregation of sh.e.l.ls of Mollusca, or fragments of the testaceous coverings of Foraminifera and other Zoophytes--nay, that they were secreted from the water itself, and then a.s.similated by these minute creatures, and that this would appear to have been the great object of their creation in such myriads. Whoever will take the trouble to observe, and reflect on what he observes, will find all his doubts vanish. If chalk be examined with a microscope, it will be found to be composed of the remains of numerous Zoophytes, of minute and divers kinds of sh.e.l.ls, and, above all, of Foraminifera, so small that their very minuteness seems to have rendered them indestructible. A hundred and fifty of these small beings placed end to end, in a line, will only occupy the s.p.a.ce of about one-twelfth part of an inch.
[Ill.u.s.tration: Chalk under the Microscope.
Fig. 126.--Chalk of Meudon (magnified).]
Much of this curious information was unknown, or at least only suspected, when Ehrenberg began his microscopical investigations. From small samples of chalk reduced to powder, placed upon the object-gla.s.s, and examined under the microscope, Ehrenberg prepared the designs which we reproduce from his learned micrographical work, in which some of the elegant forms discovered in the Chalk are ill.u.s.trated, greatly magnified. Fig. 126 represents the chalk of Meudon, in France, in which ammonite-like forms of Foraminifera and others, equally beautiful, appear. Fig. 127, from the chalk of Gravesend, contains similar objects.
Fig. 128 is an example of chalk from the island of Moen, in Denmark; and Fig. 129, that which is found in the Tertiary rocks of Cattolica, in Sicily. In all these the sh.e.l.ls of Ammonites appear, with cl.u.s.ters of round Foraminifera and other Zoophytes. In two of these engravings (Figs. 126 and 128), the chalk is represented in two modes--in the upper half, by transparency or transmitted light; in the lower half, the ma.s.s is exhibited by superficial or reflected light.
[Ill.u.s.tration: Chalk under the Microscope.
Fig. 127.--Chalk of Gravesend. (After Ehrenberg).--Magnified.]
Observation, then, establishes the truth of the explanation we have given concerning the formation of the chalky or Cretaceous rocks; but the question still remains--How did these rocks, originally deposited in the sea, become elevated into hills of great height, with bold escarpments, like those known in England as the North and South Downs?
The answer to this involves the consideration of other questions which have, at present, scarcely got beyond hypothesis.
[Ill.u.s.tration: Chalk under the Microscope.
Fig. 128.--Chalk of the Isle of Moen, Denmark.]
[Ill.u.s.tration: Chalk under the Microscope.
Fig. 129.--Chalk of Cattolica, Sicily (magnified).]
During and after the deposition of the Portland and Purbeck beds, the entire Oolite Series, in the south and centre of England and other regions, was raised above the sea-level and became dry land. Above these Purbeck beds, as Professor Ramsay tells us [in the district known as the Weald], ”we have a series of beds of clays, sandstones, and sh.e.l.ly limestones, indicating by their fossils that they were deposited in an estuary where fresh water and occasionally brackish water and marine conditions prevailed. The Wealden and Purbeck beds indeed represent the delta of an immense river which in size may have rivalled the Ganges, Mississippi, Amazon, &c., and whose waters carried down to its mouth the remains of land-plants, small Mammals, and great terrestrial Reptiles, and mingled them with the remains of Fishes, Molluscs, and other forms native to its waters. I do not say that this immense river was formed or supplied by the drainage of what we now call Great Britain--I do not indeed know where this continent lay, but I do know that England formed a part of it, and that in size it must have been larger than Europe, and was probably as large as Asia, or the great continent of America.”
Speaking of the geographical extent of the Wealden, Sir Charles Lyell says: ”It cannot be accurately laid down, because so much of it is concealed beneath the newer marine formations. It has been traced about 200 miles from west to east; from the coast of Dorsets.h.i.+re to near Boulogne, in France; and nearly 200 miles from north-west to south-east, from Surrey and Hamps.h.i.+re to Beauvais, in France;”[75] but he expresses doubt, supposing the formation to have been continuous, if the two areas were contemporaneous, the region having undergone frequent changes, the great estuary having altered its form, and even s.h.i.+fted its place. Speaking of a hypothetical continent, Sir Charles Lyell says: ”If it be asked where the continent was placed from the ruins of which the Wealden strata were derived, and by the drainage of which a great river was fed, we are half tempted to speculate on the former existence of the Atlantis of Plato. The story of the submergence of an ancient continent, however fabulous in history, must have been true again and again as a geological event.”[76]
[75] ”Elements of Geology,” p. 349.
[76] Ibid, p. 350.
The proof that the Wealden series were acc.u.mulated under fresh-water conditions and as a river deposit[77] lies partly in the nature of the strata, but chiefly in the nature of the organic remains. The fish give no positive proof, but a number of Crocodilian reptiles give more conclusive evidence, together with the sh.e.l.ls, most of them being of fresh-water origin, such as Paludina, Planorbis, Lymnaea, Physa, and such like, which are found living in many ponds and rivers of the present day. Now and then we find bands of marine remains, not mixed with fresh-water deposits, but interstratified with them; showing that at times the mouth and delta of the river had sunk a little, and that it had been invaded by the sea; then by gradual change it was lifted up, and became an extensive fresh-water area. This episode at last comes to an end by the complete submergence of the Wealden area; and upon these fresh-water strata a set of marine sands and clays, and upon these again thick beds of pure white earthy limestone of the Cretaceous period were deposited. The lowest of these formations is known as the Lower Greensand; then followed the clays of the Gault, which were succeeded by the Upper Greensand. Then, resting upon the Upper Greensand, comes the vast ma.s.s of Chalk which in England consists of soft white earthy limestone, containing, in the upper part, numerous bands of interstratified flints, which were mostly sponges originally, that have since become silicified and converted into flint. The strata of chalk where thickest are from 1,000 to 1,200 feet in thickness. Their upheaval into dry land brought this epoch to an end; the conditions which had contributed to its formation ceased in our area, and as the uppermost member of the Secondary rocks, it closes the record of Mesozoic times in England.
[77] ”The Physical Geology and Geography of Great Britain,” by A. C.
Ramsay, F.R.S., p. 64.
Let us add, to remove any remaining doubts, that in the basin of a modern European sea--the Baltic--a curious a.s.semblage of phenomena, bearing on the question, is now in operation. The bed and coast-line of the Baltic continue slowly but unceasingly to rise, and have done so for several centuries, in consequence of the constant deposit which takes place of calcareous sh.e.l.ls, added to the natural acc.u.mulations of sand and mud. The Baltic Sea will certainly be filled up in time by these deposits, and this modern phenomenon, which we find in progress, so to speak, brings directly under our observation an explanation of the manner in which the cretaceous rocks were produced in the ancient world, more especially when taken in connection with another branch of the same subject to which Sir Charles Lyell called attention, in an address to the Geological Society. It appears that just as the northern part of the Scandinavian continent is now rising, and while the middle part south of Stockholm remains unmoved, the southern extremity in Scania is sinking, or at least has sunk, within the historic period; from which he argues that there may have been a slow upheaval in one region, while the adjoining one was stationary, or in course of submergence.
After these explanations as to the manner in which the cretaceous rocks were formed, let us examine into the state of animal and vegetable life during this important period in the earth's history.
The vegetable kingdom of this period forms an introduction to the vegetation of the present time. Placed at the close of the Secondary epoch, this vegetation prepares us for transition, as it were, to the vegetation of the Tertiary epoch, which, as we shall see, has a great affinity with that of our own times.
The landscapes of the ancient world have hitherto shown us some species of plants of forms strange and little known, which are now extinct. But during the period whose history we are tracing, the vegetable kingdom begins to fas.h.i.+on itself in a less mysterious manner; Palms appear, and among the regular species we recognise some which differ little from those of the tropics of our days. The dicotyledons increase slightly in number amid Ferns and Cycads, which have lost much of their importance in numbers and size; we observe an obvious increase in the dicotyledons of our own temperate climate, such as the alder, the wych-elm, the maple, and the walnut, &c.