THE BRISTOL COALFIELD. 



27 



I now propose to begin with the Millstone grit, which lies at the 

 base of the coal measures, and take in ascending order the chief 

 deposits of stone in the coalfield for examination. 



Of the Millstone grit five specimens were selected from Brandon 

 Hill ; the following are the analyses : — 



1C0-22 



160-40 



100-52 



We gather from the detailed analysis that the alumina and lime 

 exist in a soluble form, but most of the iron is quite insoluble in strong 

 acid. We may, therefore, conclude that it is in combination with 

 silica. 



The analysis has shown that the grit contains 97'54 of silica, taking 

 the average of the five analyses ; and the microscopic sections show 

 that the grains exist in sub-angular form. The question may be 

 asked to what is the compactness of the rock due ? My late friend, 

 Mr. Stoddart. F.G.S.,* of Bristol, considered that the grains were 

 " agglomerated with oxide of iron." The available quantity of iron 

 present, however, is small, (that which is soluble in acid,) and 

 even if the alumina and lime acted in the same capacity there 

 would only be 0-63 per cent, of cementing matrix ; and it 

 seems almost impossible that so hard a rock could be cemented 

 together, as it were, with so small a portion. It is, of course, 

 possible that these ingredients may, to a degree, so act ; but I am 

 nevertheless persuaded, that not only in the Millstone grit, but in 



* Proceedings of the Bristol Naturalists' Society, Vol. 1., fart 3, new series, 

 p. 337. 



