60 



Till BftlSTOL rnu.KIKI.h. 



of rock: the Doxall grit. Mr instance, is u Pennant so tar as chemical 

 composition aud lithological features show. For the first 50 feet of the 

 Pennant it is a very coarse stone, and contains 90 per cent, of silica. 

 Following this a hetter class of rock commences, viz.. that which is so 

 extensively worked round Bristol for building and ornamental purposes. 



Analyses of tuk Pennant Ghit. 



Reviewing the whole of the analyses given of the grits, we appear 

 to have two varieties of arenaceous rocks, viz., the Millstone grit type 

 and the Pennant, and through the whole of the series of coal-measures 

 we get a repetition of these. The Pennant is lithologically distin- 

 guished by a blue colour, though at times a bod is met with strongly 

 coloured with sesqui-oxide of iron, which imparts a red tinge, aud is 

 sometimes micaceous, which I have not observed to be the case with 

 the other grits. 



It is then at the risk of serious error that we attempt to judge of 

 horizons by the lithological features of these rocks, and I take it that 

 what will apply to the Bristol Coalfield will also to others. The pro- 

 portion of silica in the Hard Venture aud Holmes grits seems to be 

 constant in typical beds of the rocks,* and even in that of the Millstone 

 grit. This I consider may be used as a test, especially over a small 

 area, but no great amount of reliance can be placed in it apart from 

 other evidence.! 



The mention of mioa in the Pennant variety will raise surprise at 

 the non-mention of alkalies in my analysis. I can only say that I have 

 failed to detect them, and I account for their absence by supposing 

 that these rocks have all been subjected, to the action of carbonic acid 

 (the percentage of carbon which they contain is a proof of this.) This 

 being so, the alkalies -would be the first to be attacked, forming 

 soluble carbonate, and, as the rocks are very porous, they would be 

 removed in solution. 



Reverting now to the changes which take place when the grits are 

 in contact with carbonaceous matter, the result would appear to be 

 that the percentage of silica is decreased, and of alumina increased, 

 while manganese, sulphur, and phosphorus appear. With a view of 



• It is, of coarse, most important that only typical specimens of the rock 

 be examined in such a tent un this. 



♦ As tho Hard Venture and Holmes erit contain tne same proportions of silica, 

 irse other means would have to be adopted to distinirniBii one from the other. 



