184 Dr. Wallich on the Origin of the 



(in other words, as typical flint) : see an invlauable paper by- 

 Mr. Graham, " On the Colloidal Properties of Silicic Acid and 

 other Colloidal Substances," Proc. Roy. Soc. for June 2, 

 1864, p. 335, where nearly the whole of the changes and 

 processes I have described, although not with relation to deep- 

 sea siliceous deposits, will be found most lucidly set forth. 



Mr. Sollas alludes throughout his paper, with one exception 

 to be hereafter mentioned, to the flints as being composed of 

 '' crystalline silica.'''' This expression strikes me as being so 

 remarkable that I must quote some of the passages in which 

 it occurs. Thus, at p. 445, " The silica of the sponge-skele- 

 tons occurs in conjunction (or probably in combination) with an 

 organic basis known as spiculin ; on solution it is liberated 

 from the spiculin, and exists in a colloid state, whence it 

 readily passes into the pectous condition, and subsequently 

 becomes hyaline ; it is, moreover, probable that, under condi- 

 tions not yet investigated^ a solution of colloid silica may give 

 rise directly to silica in a crystalline form." Again, at 

 p. 455, alluding to the silicified Blackdown shells, he says : — 

 '' The crystalline silica, which the percolating water carries in 

 solution, passes through the shell, and in some cases, under 

 favourable conditions, crystallizes out in long fine prisms." 

 At p. 456: — '' Thus the crystalline state of tliej^i?'f nodules 

 offers us no evidence for or against the theory of the formation 

 of these [the Blackdown] fossils." And ^'from this process of 

 reasoning we conclude that colloidal silica has the power of 

 changing, in course of time, into a static or crystalline condi- 

 tion." And, lastly, reverting to p. 445, from which the first 

 of these extracts was taken, we find the " crystalline " view 

 thus emphasized : — " If it be objected that in this expanded 

 explanation fact and conjecture are mixed togethei', I to some 

 extent admit it, but at the same time remark that there is no 

 conjecture in the statement that the silica which passes into 

 solution is very different from the silica which has passed 

 out of solution. The one may be conveniently called 

 organic, and the other mineral silica ; the properties of the 

 two are strikingly different ; and the process which has really 

 happened has been a solution of organic silica and a deposi- 

 tion of mineral silica, not a solution and deposition of the 

 same kind of silica." The last truisms are quoted only be- 

 cause, as the entire passage stands, the term " mineral 

 silica " would seem from the context to be a convertible term 

 for " crystalline silica^ 



On the other hand, I subscribe to the opinion which, if I 

 mistake not, is very generally entertained by chemists, that 

 the fiints are neither perfectly pure, nor, under any circum- 



