5 
Pi] 
94 Mr. HK. 'T. Harpaan—On the Cheiical Composition of Chert. 
soluble gases, &e. In every way similar to this action is that by which the more 
insoluble silica is deposited in place of carbonate of lime in the formation of chert. 
The views just stated are confirmed further by the fact, that in lists of pseudo- 
morphs by displacement, the pseudomorphs are always the more insoluble bodies, 
silica (quartz, chaleedony and hornstone) especially, is never replaced by a mineral 
more soluble in water, but is frequently shown to replace calcite and the allied forms.* 
Origin of the Nodular form of some Chert layers.—Although the cherty layers 
conform generally to the lines of bedding of the limestone, they usually occur in a 
more or less lenticular and nodular form. This may be explained on the 
supposition, that although the calcareous matter forming the bottom of the 
carboniferous sea would be in a somewhat generally pasty condition, some parts 
would be more soluble in sea water than others. A small patch might thus be 
easily altered and more or less replaced by silica, whilst surrounding portions would 
be nearly or quite unaffected. Molecular attraction might also in such cases 
determine the deposition of silica near the first particles deposited. 
It would also depend much, both on the homogeneity of the limestone and the 
rate of its deposition, whether thick and regular beds or merely nodules and patches 
of chert were formed, but it appears clear that any given bed of chert must have been 
formed before the overlying bed of limestone was formed, for it always conforms to 
the bedding, and is never found in cracks or joints. 
Source from whence the Silica was obtained.—Professor Hull has referred to the 
physical conditions of the period, and shown that the sea gradually became 
shallower, while the supply of silica as denoted by the predominance of chert in the 
upper beds must have increased. It may not be out of place here to point out 
that all the high ground surrounding the Irish carboniferous sea was composed of 
highly siliceous and felspathic rocks. Oy the west, the granitic and metamorphic 
rocks of Galway and Mayo; on the north, those of Donegal and Tyrone ; and 
on the east, the Wicklow granites. The temperature of the time was high, and 
the atmosphere contained a much greater proportion of carbonic acid, than that 
of the present day. Under these circumstances the decomposition of such rocks 
would go on rapidly. The silicates would be quickly attacked by carbonic acid, 
the bases removed as soluble carbonates, while a quantity of silica (large compared 
to that which would be taken up at the present day) would be dissolved and 
carried down into the sea by river waters. As Dr. T. Sterry Hunt has shown, a 
very large amount of silica in solution is carried into the sea by rivers at the 
present time, and we are entitled to believe that a much greater amount was 
dissolved under the more favourable conditions existing during the carboniferous. 
period, while with a sea gradually shallowing and consequently a more perceptible 
evaporation—the silica so brought down would be the more likely to substitute 
itself for the carbonate of lime. 
* See Brooke and Miller’s Ed. of Phillips’ Mineralogy—List of Pseudomorphs, pp. 100-108. 
