GYMNOSPERiYLE. 181 



it to represent Globigerina-ooze, 1 to have been directly deposited on a former land surface, 

 and we consequently find that it is invariably preceded by some more littoral sediment. 

 The nearer the original centre of depression or focus of subsidence, the older the Green- 

 sands and Gaults must necessarily be ; and the farther we recede from it in any landward 

 direction, the newer they will be. Now, apart from physical evidence, a comparison of 

 the Faunas of our Chalk with those of any European bed correlated with it to the 

 eastward would at once show that if one were older than another, it would be that of our 

 area. Eorms like Mosasaurus, which only appear in our very latest Chalk deposits, 

 abound in Cretaceous deposits of more central Europe ; whilst others, such as Ichthyosaurus, 

 found abundantly in our Chalk-marl are, on the contrary, absent in them. The rapid 

 increase in the proportion of long-canaled and other Eocene-looking Gastropods in 

 the Chalk, as we recede from Kent and Sussex, reaches a maximum in the Danish 

 Upper Chalk, and indicates most conclusively a more and more recent period of deposition 

 for the beds in which they occur. The littoral and each subsequent zone must in 

 fact have been constantly travelling outward and forward, accumulating only until 



1 True Chalk is a pure white limestone, composed of the remains of Foraminifera, valves of Ostracorla, 

 excessively minute Coccolliths, shell prisms of Inocerami, Sponge spicules, and other debris of organic life. 

 It was, until recently, admitted to be a truly oceanic deposit, of similar nature to Globigerina-ooze ; but 

 Mr. Wallace, supported by the late Dr. Gwyn Jeffreys, has put forward the view that it was formed in 

 shallow water (' Island Life.') Its vast extent, homogeneous nature, and freedom from terrestrial impurity, 

 show that it must have been formed remote from land, while its larger organisms, mainly Echinoderms 

 and Sponges, are, with some exceptions, such as are now met with in abyssal depths. Mr. Wallace laid some 

 stress on the difference in composition of fresh Globigerina-ooze and Chalk, as shown by analysis ; but Mr. 

 Murray has recently stated that the percentage of carbonate of lime varies from 40 to 95 in the ooze. The 

 comparison took no account of the fact that the Chalk had been elevated for ages, during which it has 

 acted as a sponge for the collection and percolation of rainwater charged with carbonic acid, which has 

 been ceaselessly removing some of its original constituents. Its silica has been dissolved and re-precipitated 

 as flint, its iron has been segregated into crystalline masses, its manganese into dendritic markings, 

 siliceous sponge skeletons have been dissolved and replaced by calcite, calcite shells by silica, and 

 aragonite shells removed entirely. Layers of Chalk a foot in thickness have been reduced to an inch by 

 the removal of lime in solution. The late Dr. Gwyn Jeffreys had not studied the question, and based his 

 conclusions upon the Mollusca only, and these chiefly of the Chalk-marl, and he seemed unaware that only 

 the calcite shells remained in true Chalk. Of these shells Terebratula, Pecten, Lima, and Spondylus 

 are the chief genera still existing, and all but the last are already known to inhabit water 1400 fathoms in 

 depth. Moreover, if the Chalk sea did not communicate with the Arctic Ocean, as Prof. Prestwich and 

 others believe, and was shut off from the Antarctic by land between Africa and South America, as 

 there is also much evidence to support, its abyssal depths would have been warm instead of icy cold, 

 and its former abyssal inhabitants, accustomed to warmth, would have sought shallower water in order to 

 find an equal temperature, and become, as Gwyn Jeffreys states them to be, a tropical assemblage at the 

 present day. The blue and green muds of the ' Challenger ' pass into Globigerina-ooze with an increased 

 depth, and their equivalents of Gault and Greensand pass into Chalk in exactly the same way. The 

 alternative theory of Wallace, that Chalk is decomposed coral mud, could not have been advanced by a 

 geologist, as while it contains some well-preserved solitary corals, not a trace of a reef-building coral has 

 ever been met with either in or surrounding it, nor even in any contemporaneous deposit. 



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