172 Dr. Wallich on the Origin of the 



and is, as I stated in ray former paper, as indispensable 

 a factor in the production of the flints, as they now present 

 themselves in the Upper Chalk, as the silica itself which is 

 derived from the sponge-spicules. In the other region 

 sponge-life did, no doubt, occur to a certain and, possibly, 

 considerable extent. But the condition of aqueous movement 

 at the sea-bed during the deposition of the Trimrainghara 

 beds must there have constituted an insuperable obstacle (as it 

 undoubtedly is to this day, at depths no greater than those 

 determined for the Trimmingham beds) to the development of 

 both the sponges and their protoplasmic nidus in sufficient 

 abundance to lead to the formation of the typical black flint, 

 which, according to my hypothesis, is as distinct in its mode 

 of formation from the cherty varieties as the chert is distinct 

 in its mode of formation from the chalk. Accordingly, the 

 element of depth becomes a material factor in our present 

 investigation. 



Professor Prestwich, when referring to " Submarine Tempe- 

 ratures" *, in his Address delivered in 1871 at the Anniversary 

 Meeting of the Geological Society, observed : — " From these 

 considerations the question arises whether the deep sea in 

 which the Chalk was deposited may not also have been a sea 

 shut out from direct communication with the Arctic seas " [lac. 

 cit. p. 39). . . . " I think, therefore, that the hypothesis with 

 regard to the continuity of that sea-bed (the Post-cretaceous 

 Atlantic) from the period of the chalk to the present is one of 

 high probabihty " {ihid. p. 41). And again : — " The Chalk, 

 attaining as it does a thickness of 1000 to 1500 feet, was 

 always looked upon by geologists as the deposit of a very deep 

 sea " {ihid. p. 46). Mr. Whitaker, in his excellent ' Guide to 

 the Geology of London ' (The Geological Survey of England 

 and Wales : 1875), says : — " By its fossils the Chalk is proved 

 to be the deposit of a deep sea — a deposit of much the same 

 character as that now forming in the mid- Atlantic, and which, 

 like the Chalk, is largely made up of the remains of microscopic 

 Foraminifera " {op. cit. p. 19). And, lastly (though many 

 additional authorities to the same eflfect might be cited). Pro- 

 fessor Martin Duncan, during the discussion which followed 

 the reading at the Geological Society of Mr. Sollas's own paper 

 " On the Markings in the Chalk of the Yorkshire Wolds," 



• See the elaborate and admirable memoir entitled '^ Tables of the 

 Temperatures of the Sea at different depths, reduced and collated from 

 the various observations made between 1749 and 1868, with maps and 

 sections. By Joseph Prestwich, M.A., F.R.S., «&c.,' Phil. Trans. Roy. 

 Soc. vol. 165, pt. 2, 1874. 



