t 
56 Dr. Wallich on the Identity of the Chalk 
/ Now the quantity of silica in the recent mud has been 
“variously estimated at from 20 to 30 per cent. of the mass, 
the carbonate of lime at from 50 to 60 per cent. All I now 
profess to accomplish is—to show that my results, as deducible 
from analyses of the material obtained from hermetically closed 
cavities, correspond very much more closely with the percent- 
ages found in the recent calcareous mud than those erroneously 
supposed (as I contend) to represent the original quantity of silica 
_contained in the chalk.| That I have done this much will, I 
“think, be freely conceded. But in order sti!l more firmly to 
establish my position, I must have such a detailed qualitative 
as well as quantitative analysis made of the quantity of car- 
bonate of lime which is undoubtedly locked up in the cherty 
portion of the contents, and rendered temporarily insoluble 
by its combining with the silica to form the large residue 
included in my analysis under the head of ensoluble matter. 
There is another point that demands explanation. I have 
had to balance the percentages of material, as closely as I con- 
sidered justifiable in the absence of further analytical data, in 
quite an opposite direction—that is to say, by showing that 
this large insoluble residue is not pure silica but chert, and that 
the chert found in the hermetically closed nodular cavities, and, 
indeed, in “ the flint” generally, is here as elsewhere a com- 
pound of organic silica and lime, which unite, where an 
organic colloid is present, in almost indefinite proportions. 
Now the silica in the recent calcareous mud does not exhibit 
any appreciable trace of intermixture with carbonate of lime. 
In the contents of the nodular cavities the captive silica and 
lime have, during the lapse of ages of undisturbed seclusion 
(the existing conditions being precisely those most likely to 
favour their union), become closely combined,—the point at 
which any further combination between the materials was 
arrested having been the point at which they became anhydrous. 
The chert of the flints is nearly altogether insoluble in 
hydrochloric acid. The most perfect form of chert met with 
show the percentage of sz/zca in seven samples of chalk which were taken 
by me at various levels from the face of a lofty cliff in one of the pits at 
Charlton :— 
per cent. 
JA’ GPS oe MERCER O TY ERO chs 0:57 
HSM Melita stain ele (ulavetaceiee 0:50 
(OF igh id eRe ATRL en 0:49 
1) sb ch Che EMRE ti 0-48 
TEU Name Rerste eas ket fee ieiezolereae 0:64 
SNA eye Rade Mc uel aiale Folate 0:95 
