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PART II. 
THE CHEMICAL COMPOSITION OF CHERT, AND THE CHEMISTRY 
OF THE PROCESS BY WHICH IT IS FORMED. By EDWARD 
T. HARDMAN, r.cs. 
The following twelve analyses of specimens from amongst those examined 
microscopically by Professor Hull, throw much light upon the origin of Chert. 
It will be observed that the composition varies from that of a siliceous limestone 
to that of an almost wholly siliceous rock. I have been unable to find any 
published analysis of this mineral, except one given by Dana of a gray chalcedonic 
hornstone from Marienbad,* the composition of which resembles that of some of 
the more siliceous varieties I have examined ; but neither the formation in which 
it is found, nor the mode of its occurrence are mentioned—this and the state-- 
ment by Cottat and others that Chert consists mainly of silica, with but traces of 
impurities in the shape of other constituents is about all that is to be found in the 
mineralogical text-books with regard to it. Dana says, “Chert is a siliceous 
stone containing some lime ;” while Gmelin classes hornstone amongst the crys- 
talline varieties of Chert. 
The numbering of the specimens corresponds with that used in the section on 
the microscopic examination. It should be mentioned that many of the specimens 
contained strings and veins of calcite of later ovigin—care was taken to select 
portions for analysis perfectly free from these. The carbonate of lime noted below 
in the analyses is undoubtedly an original constituent. 
The analyses were conducted as follows :—Two analyses of each specimen were 
made. (a) The powdered chert was boiled with hydrochloric acid and allowed to 
remain in it one night. The solution gave the soluble silica and the carbonates, 
together with some iron oxide (chiefly protoxide) and alumina. (b) A fresh portion 
of the chert, or in some cases the residue from (a) was fused with the alkaline 
carbonates, giving the insoluble silica, with alumina, ferric oxide, and sometimes 
lime and magnesia; column (0) in the tabulated analysis represents in each case 
the analysis of the insoluble residue of (a) minus the organic matter, &c.[ The 
third column gives the total analysis of the specimen made up from (a) and (6). 
* System of Mineralogy (1873), p. 195. 
+ Rocks classified and described. See also Dana’s Manual of Mineralogy. Nicol’s Elem. of 
Mineralogy. Watt’s Chem. Dict. Uve’s Dict. Sci. and Art. Gmelin’s Hand-book of Chemistry, &c. 
{These were determined in other portions of the rock, but for convenience I have placed them in 
column (@). 
