456 Mr. W. J. Sollas on the Flint 



thought, but at work from the very beginning; and the crystals 

 commence all along the boundary of the shell, from which the 

 silicated water oozed out. From this process of reasoning 

 we conclude that colloidal silica has the power of changing, 

 in course of time, into the static or crystalline condition. In 

 the case of the Blackdown shells the colloidal silica probably 

 remained for a long time in a jelly-like condition, which may, 

 among other things, help to account for its perfect crystal- 

 lization. 



" Thus the crystalline state of flint noludes offers us no 

 evidence for or against our theory of the formation of these 

 fossils. This theory may be summed up under two heads : — 

 (1) combination of silicic acid with animal matter of various 

 kinds — a chemical fact ; and (2) concentration of the silica 

 from the silicate of animal matter thus formed, by the extri- 

 cation of the organic part of the compound. This is a pure 

 assumption, but one which agrees very well with other well- 

 known facts in chemistry." 



That organic matter has in certain cases been replaced by 

 silex may be considered certain, the numerous observations 

 made from the time of Von Buch and Bischoff down to the 

 present day seem to leave no doubt on this point ; the occur- 

 rence of silicified wood is an instance ; and still more striking 

 instances are known, as that of the nuts so often mentioned, 

 in which the soft kernels have been converted into silex, while 

 theshell remains unchanged — or, better still, that of the marvel- 

 lous silicified Trigonice from the Portland beds of Tisbury, 

 Wilts, described and figured by Mr.Charlesworth as still show- 

 ing the structure of the animal, even to the separate filaments 

 of its branchiae ; and if, in these cases, silicification of organic 

 matter has occurred, it certainly might in that of the chalk- 

 flints ; but whether, as a matter of fact, it has so assisted in the 

 formation of these bodies, is quite a different question ; and 

 what little evidence we can find bearing directly on the point 

 seems to show that it has not. Some little light is thrown on 

 the subject by the condition of the sponge enclosed in the 

 flint. Very frequently it consists of a network, the interstices 

 of which are empty and not filled with flint, the solid flint 

 forming a complete enclosure to the sponge, but stopping 

 short internally at the borders of the skeletal network, just 

 where animal matter might be expected to have been most 

 abundantly present. If we try to elude this difficulty by sup- 

 posing the sarcode to have been already shed over the sub- 

 jacent ooze, then all connexion is lost between the form of 

 the sponge and that of the enveloping flint ; or, again, if we 

 suppose the sponge to have been completely covered by the 



