580 , KEPOBT— 1880. 



The chief deposits of rock in the coalfield were then referred to- in ascending- 

 order, commencing -with the Millstone grit. Several samples of this, taken from 

 Brandon Hill, gave from 97'4 to 98-5 per cent, of silica. In places it is used for 

 Irick-making, being mixed -with the other material to increase the proportion of 

 silica. It -was pointed out that there -were other heds higher up in the coal- 

 measures -which -woidd do equally -well, and in some cases better, for this purpose. 

 The paper next referred to the ' Pennant grit.' There is considerable difficulty in 

 defining the limits of this deposit, but it was certainly not 2000 feet thick, as some 

 authors had stated. The paper places the thickness at about 970 feet ; but the 

 middle or Pennant series of coal measures, so called on account of the Pennant 

 being so extensively developed in this division, was about 2000 feet thick, and 

 this, probably, was the origin of the mistake regarding the thickness of the Pennant. 



The 970 feet of rock above referred to as tJie Pennant grit, was only entitled to 

 that name as a local distinction. It was nothing more than an extraordinary de- 

 velopment of a coal measure grit; the ' Doxall grit' of the lower series, for 

 instance, was quite as much a Pennant, if that term is to distinguish a certain class 

 of rock. 



After a careful examination of the rocks of the coalfield, the author had come 

 to the conclusion that, owing to the great similarity of Carboniferous arenaceous 

 rocks, occurring at diflerent horizons, it was at the risk of serious error to rely 

 upon them for correlation or stratigraphical landmarks. The proportion of silica 

 could be sometimes used as a guide in determining one from another, but little 

 reliance could be placed on it over a large area, as so many contained nearly about 

 the same amount. 



The author's analysis showed the first 50 feet of the Pennant to contain 90 per 

 cent, of silica ; but after this, for a considerable thickness, the proportion varied 

 from 84 to 89 per cent. 



The paper then referred to changes in the grits when in contact with carbo- 

 naceous matter. The author found that the proportion of alumina increased to 

 the whole, and this mostly as a silicate. By comparing the analysis of duns and 

 shale from the district -with that of these rocks, the same constituents were found to 

 be present, the great difference being in the greater proportion of alumina in duns 

 and shale. As a rule, the latter beds always overlie seams of coal ; but in cases 

 where rock followed, the author found that as it neared the coal it became more 

 fissile and argillaceous. 



This change was ascribed to the action of carbonic acid gas, generated by de- 

 composing vegetation on silicates. The analysis of the rocks given showed them 

 to have been formed from the denudation of older silicate rocks, and the action of 

 carbonic acid on such sediment would be to readily decompose all silicates with 

 the exception of silicate of alumina, which would thus increase in proportion to 

 the whole, and give rise to beds of the composition of duns and shale.* To this 

 cause the author attributed the formation of the latter deposits, and contended 

 that where they occur apart from carbonaceous matter is no proof that it was 

 never there, and destroyed by decomposition. 



SATURDAY, AUGUST 28. 



The Section did not meet. 



' The author was not dealing with the hydrocarbons which shales sometimes 

 contain. 



