OCCURRENCE OF CHERT OF ORGANIC ORIGIN. 847 



SILICA FAMILY. 



The silica deposits are all included under the general term chert. 

 This term, as here applied to siliceous layers and formations, includes 

 amorphous, partly amorphous, and crystallized silica. Often in the same 

 mass may be seen very minutely spotted quartz grains separated by partly 

 amorphous material, cryptocrystalline silica of various kinds, such as 

 chalcedony, and completely crystalline quartz. In almost every extensive 

 chert formation all these varieties are mingled in the most complex manner. 

 Chert comprises both organic and chemical deposits. 



The most extensive deposits of chert are those of organic origin, the 

 silica being taken by certain animals and built into their hard parts, 

 precisely as is calcium carbonate by other animals. The animals which 

 absorb the greatest amount of silica are sponges of a certain class. Other 

 than the sponges, radiolaria and diatoms appear to be the more important. 

 These and other animals may build up considerable deposits of chert. 

 As illustrations of chert formations of organic origin may be mentioned 

 the Cretaceous flints described by Wallich, a the flints of the Trimmingham 

 chalks described by Sollas, 6 and extensive deposits of chert in Ireland, 

 England, Wales, Spitsbergen, and Axels Island, described by Hinde." 

 Hinde says that the chert beds in Yorkshire have an estimated thickness 

 of about 30 meters, and in North Wales of 100 meters, and these, he says, 

 can be proved to be due to sponge remains. 1 * 



But probably the most extensive formation of chert which is known to 

 be of organic origin is that of Axels Island, where the beds of chertv 

 material, according to Hinde, aggregate 260 meters in thickness." This is 

 not a continuous mass of chert, but consists of a number of beds inter- 

 stratified with limestone. 



a Wallich, G. C, A contribution to the physical history of the Cretaceous flints: Quart. Jour. Geol. 

 Soc. London, vol. 36, 1880, pp. 68-91. 



» Annals Mag. Nat. Hist., 5th ser., vol. 6, 1860, p. 438. See also Walcott, C. D., Fossil medusa?: 

 Mon. U. S. Geol. Survey, vol. 30, 1898, p. 18. 



c Hinde, G. J., On the organic origin of the chert in the Carboniferous limestone series of Ireland, 

 and its similarity to that in the corresponding strata in North Wales and Yorkshire: Geol. Mag., 

 London, new ser., dec. 3, vol. 4, 1887, pp. 435, 436. Hinde, G. J., On the chert and siliceous schists 

 of the Permo-Carboniferous strata of Spitsbergen, and on the character of the sponges therefrom, 

 which have been described by Dr. E. von Dunikowski: Ibid., vol. 5, pp. 241-251. 



''Hinde, cit., vol. 4, pp. 435-446. 



f Hinde, cit, vol. 5, p. 243. 



