HISTORY OF THE CRETACEOUS FLIJ^TS. 69 



quartz of granitic and other plutonic rocks- dissolved in the heated 

 water and erupted into the hasin of the Chalk formation. 



There are other observers, however (Mr. Mantell goes on to say), 

 who are inclined to believe that the flints in the Chalk, and also in 

 the Portland Oolite and other calcareous deposits, owe their origin to 

 the Sponges, of which such frequent traces are recognizable in con- 

 nexion with the flint. Mr. Bowerbank, he says, advocates this origin 

 for every kind of flint nodule and vein in the Chalk. But how 

 or why the silex, thus considered as representing the Sponges, should 

 have invested or replaced organic bodies, and the Porifera in par- 

 ticular, Mr. Mantell contended was left wholly unaccounted for. 

 Prof. Ehrenberg, he said, suggested that the flints may be due to the 

 chemical segregation of silex derived from the siliceous remains of 

 Diatomacece, Polycystina, &c. 



In ' The Student's Elements of Geology ' (the latest of Sir Charles 

 Ly ell's works in which the subject of the flints is touched upon)* it 

 is stated that the origin of the layers of flint, whether in the form of 

 nodules or continuous sheets, or in veins or cracks not parallel to 

 the stratification, has always been more difficult to explain than that 

 of the White Chalk. But here, he says, the late deep-sea soundings 

 have suggested a possible source of such mineral matter. According 

 to Dr. Wallich it was ascertained that, while the calcareous Glohi- 

 gerince had almost exclusive possession of certain tracts of the sea- 

 bottom, they were wholly wanting in others, as between Greenland 

 and Labrador. " But in several of the spaces where the calcareous 

 Rhizopods are wanting, certain microscopic plants called Diatomacece, 

 the solid parts of which are siliceous, monopolize the ground at a 

 depth of nearly 400 fathoms, or 2400 feet. The large quantities of 

 silex in solution," Sir Charles then proceeds to say, " required for the 

 formation of these plants may probably arise from the disintegration 

 of felspathic rocks. As more than half of their bulk is formed of 

 siliceous earth, they may afford an endless supply of silica to all the 

 great rivers that flow into the ocean. We may imagine that after 

 the lapse of many years, or centuries, changes took place in the 

 direction of the marine currents, favouring at one time in the same 

 area a supply of siliceous, and at another of calcareous matter 

 in excess, giving rise in the one case to a preponderance of Glo- 

 higerince, and in the other of Diatomacece. These last, and certain 

 Sponges, may by their decomposition have furnished the silex which, 

 separating from the chalky mud, collected round organic bodies, or 

 formed nodules, or filled shrirkage-cracks"t. 



Again, in his ' Principles of Geology,' the same distinguished 

 author says, " The homogeneous character of the White Chalk or upper 

 portion of the great Cretaceous formation throughout a large part of 

 Europe is now (1872) explained by discovering that it is made up 

 exclusively of the remains of the calcareous shells of Eoraminifera ; 

 ivhile the siliceous 'portion has been derived chiefly from plants called 

 Diatoms" +, 



* ' The Student's Elements of Geology,' 1871, p. 264. t Op. cit p. 265. 



X Lyell's ' Principles of Geolog)',' 11th edit. (1872), vol. i. p. 216. 



