MIDDLE CAMBRIAN. 19 



which those preserved are but a remnant, He describes the accumulation 

 of sponge spicules and their solution, and summarizes on the latter as 

 follows: 1 



1. Fossil sponge spicules are frequently eroded externally, and their axial canals 

 enlarged internally; 2, all flesh spicules, necessarily once present, have entirely dis- 

 appeared ; 3, in many chalk flints Veutriculite and Lithistid skeletons occur, perfectly 

 preserved as to form, but not as solid network, merely as empty casts ; 4, the skeletons 

 of many fossil sponges have exchanged a siliceous for a calcareous composition. 



The mode of solution of the spicules is one of great interest in this 

 connection, and Professor Sollas quotes Julian, where he suggests that the 

 humus acids produced dming the submarine decomposition of organic 

 matter may have been the agents which accomplished the solution. He 

 says: "This may very possibly have been the case, though possibly the 

 water at the sea bottom may, even without the assistance of these substances, 

 have been a sufficiently powerful solvent." 



In reference to the redeposition of silica, Professor Sollas considers 

 that there are three different modes by which the deposition may have been 

 effected: First, simple deposition; second, deposition as pseudomorph after 

 carbonate of lime; and third, deposition in combination with bases, forming 

 silicates. Under the first, he describes deposition by simple crystallization 

 of quartz from siliceous solution, which disposes of the view that the 

 presence of an organic nidus is necessary. The deposition of silica as a 

 pseudomorph after carbonate of lime he considers to be the method of 

 formation of flint and chert. Some of the Trirnmingham flints consist 

 within of ordinary flint, black, translucent, and compact, but exteriorly 

 simply of ordinary chalk, with a few siliceous remains scattered through 

 it. Between these two he found every intermediate stage of silicification. 

 He says: 3 



Passing from the chalk to the flint, one finds first the coccoliths, foraminifers, 

 and other calcareous constituents of the chalk converted into silex, the siliceous 

 pseudomorphs retaining all the details of their original form down to the delicate 

 strioe on some of the foraminiferal tests; from the mixture of chalk material and its 

 siliceous pseudomorphs, we proceed nearer the flint and reach a porous superficial 

 layer, formed by the cementation of the siliceous pseudomorphs together into a sili- 

 ceous network; the side of this network next the flint enters half immersed into it, as 

 it were; a step further and we reach the flint itself, the siliceous pseudomorphs being 



1 Annals Mag. Nat. Hist., 5th series, Vol. VI, 1860, p. 443. * Loc. cit., p. 447. 



UJ L I 8 R '<■ R Y 



