| OCR Text |
Show 92 ISLAND LIFE. (PART 1. and even pure sands and. sandstones, characterised by the same general types of fossil remains as the chalk itself. These beds imply the vicinity of land, and this is even more clearly proved by the occurrence, both in the Upper and Lower Cretaceous, of deposits containing the remains of land-plants in abundance, indicating a rich and varied flora. Now all these facts are totally opposed to the idea of anythincr like oceanic conditions having prevailed in Europe during t:l the Cretaceous period; but it is quite consistent with the ex-istence of a great Mediterranean sea of considerable depth in its central portions, and occupying, either at one or successive periods, the whole area of the Cretaceous formation. We may also note that the Maestricht beds in Belgium and the Faxoe chalk in Denmark are both highly coralline, tbe latter being, in fact, as completely composed of corals as a modern coralreef · so that we have here a clear indication of the source when' ce the white calcareous mud was derived which forms the basis of chalk. If we suppose that during this period the comparatively shallow sea-bottom between Scandinavia and Greenland was elevated, forming a land connection between these ·countries, the result would be that a la.rge portion of the Gulf Stream would be diverted into the inland European sea, and would bring with it that abundance of Globigerinre, and other Foraminifera, which form such an important constituent of chalk. This sea was probably bordered with islands and coral-reefs, and if no very large rivers flowed into it we should have all the conditions for the production of the true chalk, as well as the other members of the Cretaceous formation. The products of the denudation of its shores and islands would form the various sandstones, marls, and clays, which would be deposited almost wholly within a few miles of its coasts; while the great central sea, perhaps at no time more than a few thousand feet deep, would receive only the impalpable mud of the coral-reefs and the constantly falling tests of Foraminifera. These would imbed and preserve for us the numerous echinoderms, sponges, and mollusca, which lived upon the bottom, the fishes and turtles which swam in its waters, and sometimes the winged reptiles that flew overhead. The CHAP. VI.] GEOGRAPHICAL AND GEOLOGICAL CHANGES. 03 ~bundance. of. am_monites, and other cephalopods, in the chalk, IS another 1~dwatwn that the water in which they lived was not very deep, smce Dr. S. P. Woodward thinks that these orcran-isms were limited to a depth of about thirty fathoms. 0 The best example of the modern formation of chalk is perha~s to be f?und on the coasts of sub-tropical North Amenca, as descnbed in the followincr passage :- " The observations of ~ourtales sh~w that the steep banks of Bahama are covered w1th soft white lime mud. The lime~ ottom, which consists almost entirely of Polythalamia, covers m greater depths the entire channel of Florida. This formation extends without interruption over the whole bed of the Gulfstream in the Gulf of Mexico, and is continued along the Atlantic coast of America. The commonest crenera met with in this deposit ar~ G!obigerin~, Rotalia cultrat:, in large numbers, several Textilanre, Margmulinre, &c. Beside these small free corals, Alcyonidre, Ophiurre, Mollusca, Crustacea, ' small fishes, &c., are found living in these depths. The whole seabottom appears to be covered with a vast deposit of white chalk still in formation." 1 There is yet another consiueration which seems to have been altog~ther overlooked by those who suppose that a deep and open 1sla~d-studded ocean occupied the place of Europe in Cretaceous times. No fact is more certain than the considerable break, indicative of a great lapse of time, intervening between ~he Cret~ceous and Tertiary formations. A few deposits of mtermedmte age have indeed been found, but these have been generally allocated either with the Chalk or the Eocene, leaving the gap almost as pronounced as before. Now, what does this gap mean? It implies that when the deposition of the various Cretaceous beds of Europe came to an end they were raised above the sea-level and subject to extensive denudation, and that f?r a long but unknown period no extensive portion of wh1a t IS now Eu.r opean land was below the sea-level . It was on y when th1s period terminated that large areas in sev~ral parts. of Europe became submerged and received the earliest Tertiary deposits known as Eocene . If, th ere.1c ore, 1 Geological Magazine, 1871, p. 426. { |
