92 Mr. E: T. Harpman—On the Chemical Composition of Chert. 
traces of silica when treated with a strong solution of caustic potash, even after 
long continued boilings. As there can be no doubt that the silica of chert is in the 
amorphous condition, Fuch’s statement* that the amorphous silica of chalcedony and 
flint can be removed by a solution of caustic potash must be a mistaken one, 
especially as he refers opal to the anhydrous amorphous form. I think it is much 
more likely to be the hydrated forms of silica which are so affected, and this has been 
lately shown to be the case by C. Friedel, in his paper on the alteration of flints.+ 
The following points will be apparent from the foregoing analyses. 
1. That chert has no definite chemical composition, the amount of silica in 
specimens (which present but little external differences) varying from 65°15 per 
cent. to as much as 95°50 per cent., and it is not unlikely that a larger series of 
analyses would show a complete transition from ordinary limestone up to the most 
siliceous variety of chert. 
9, The silica isin the anhydrous condition, and is practically uncombined with 
bases—the amount of lime, iron, &c., combined as silicates being extremely small, 
and the principal variation in compositions is due to the residue of carbonate of 
lime remaining from the original limestone. 
3. Taken in connexion with the microscopic examination, the analyses go to show 
that the formation of chert can only be accounted for by a process of pseudo- 
morphism from limestone. ‘The varying proportions of the silica and the carbonate 
of lime—together with the presence of organisms whose shells, originally calcareous 
“now silicified, are visible in some of the most highly siliceous specimens—prove this. 
Process by which the change was effected.—The above points being true, 10 only 
remains to show how the change could have taken place. The process of pseudo- 
morphism by substitution or replacement might be defined chemically, as that by 
which any liquid holding mineral matter in solution deposits w less soluble mineral 
in order to take wp a portion of a mineral which it can more easily retain in solution. 
There may be a few exceptions to this, but I think it will be found to hold good 
in most cases, and that the pseudomorph is almost invariably a more insoluble body 
than that which it replaces. In the many experiments made by Bischof with various 
solutions on minerals, the results are altogether in accordance with this view, and it 
is thus easy to account for the occurrence of chert. Supposing that the ocean in 
which the limestone was being formed contained an unusually large per-centage of 
silica (which would mean, however, actually a very trifling proportion), it is but 
natural to suppose that the water would elect to dissolve small portions of the 
more easily soluble carbonate of lime, and in their place to deposit equivalent 
portions of the less soluble silica in the gelatinous state. 
That this would go on at the bottom of the sea there could be no reason to doubt, 
and contemporaneously with the accumulation of the limestone itself, and another 
circumstance would assist in the process—the gradual decay of the animal matter 
* J], Gmelin’s Hand-book of Chemistry, Cavendish Soc. Ed., Vol. III., 354. 
+ “On certain alterations of Agates and Flints,” Comptes Rendus Ixxxi 979; also Journ. Chem. Soe, 
Lond. Vol. I. N.S. p. 526. 
