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Show 90 ISLAND LTFE. [PART I. Mr. H. B. Brady has led him to a different conclusion; for he finds numerous forms at the bottom quite distinct from those which inhabit the surface, while, when the same species live both at surface and bottom, the latter are always larger and have thicker and stronger cell-walls. This view is also supported by the fact that in many ~tations not far f~om our own shores Globigerinre are abundant m bottom dredgmgs, but. are never found on the surface in the towing-nets.1 These orgamsms then exist almost universally where the waters are pure and are not too cold, and they would naturally abound most where ~he diffusion of carbonate of lime both in suspension and solution afforded them an abundant supply of material for their shelly coverings. Dr. Vv~ allich believes that they flourish best where the warm waters of the Gulf Stream bring organic matter frol!l which they derive nutriment, since they are wholly wanting in the course of the Arctic current between Greenland and Labrador. Dr. Carpenter also assures us that they are rigorously limited to warm areas. Now with reo-ard to the depth at which our chalk was formed, b h . we have evidence of several distinct kinds to show t at It was not profoundly oceanic. Mr. J. Murray, in the Report alrea~y referred to, says: "The Globigerina-oozes which we get m shallow water resemble the chalk much more than those in deeper water, say over 1,000 fathoms." 2 This is important and weighty evidence, and it is supported in a striking manner by the nature of the molluscan fauna of the chalk. Mr. Gwyn Jeffries, one of our greatest authorities on shells, who has himself dredo-ed largely both in deep and shallow water and who has nob theory to support, has carefully examined this question. Taking the whole series of genera which are found in the Chalk formation, seventy-one in number, he declares that they are all comparatively shallow-water forms, many living at depths not exceeding 40 to 50 fathoms, while some are confined to still shallower waters. Even more important is the fact that the genera especially characteristic of the deep 1 Notes on Reticularian Rhizopoda ; in Microscopical Joumal, Vol. XIX., New Series, p. 84. 2 Proceedi11gs of the Royal Society, Vol. XXIV. p. 532. cHAP. YI.] GEOGRAPHICAL AND GEOLOGICAL CHANGES. 91 Atlantic ooze-Leda, Verticordia, Nemra, and the Bulla family -are either very rare or entirely wanting in the ancient Cretaceous deposits.1 Let us now see how the various facts already adduced will enable us to explain the peculiar characteristics of the chalk formation. Sir Charles Lyell tells us that 11 pure chalk, of nearly uniform aspect and composition, is met with in a northwest and south-east direction, from the north of Ireland to the Crimea, a distance of about 1,140 geographical miles; and in an opposite direction it extends from the south of Sweden to the south of Bordeaux, a distance of about 840 geographical miles." This marks the extreme limits within which true chalk is found, though it is by no means continuous. It probably implies, however, the existence across Central Europe of a sea somewhat larger than the Mediterranean. It may have been much larger, because this pure chalk formation would only be formed at a considerable distance from land, or in areas where there was no other shore deposit. This sea was probably bounded on the north by the old Scandinavian hio-hlands ex- b , tending to Northern Germany and North-western Russia, where Palreozoic and ancient Secondary rocks have a wide extension, though now partially conqealed by late Tertiary deposits; while on the south it appears to have been limited by land extending through Austria, South Germany, and the south of France, as shown in the map of Central Europe during the Cretaceous period in Professor Heer's Primeval J!Vorld of Switzerland, p.175. To the north the sea may have had an outlet to the Arctic Ocean between the Ural range and Finland. South of the Alps there was probably another sea, which may have communicated with the northern one just described, and there was also a narrow strait across Switzerland, north of the Alps, but, as might be expected, in this only marls, clays, sandstones, and limestones were deposited instead of true chalk. It is also a suggestive fact that both above and below the true chalk, in almost all the countries where it occurs, are extensive deposits of marls, clays, 1 See Presidential Address in Sect. D. of British Association at Plymoulh 1877. ' |