Notices of Memoirs — Coal-fields of Pennsylvania. 523 



Intrusive Hocks. — Carn Ysgubor is formed of an intrusive mass of 

 quartz-albite-diabase, which has invaded the sediments of Lower 

 Llanvirn, Arenig, and Lingula Flags. A small intrusion occurs 

 south of Abermawr, where Lingula Flags are in contact with a 

 quartz-keratopliyre. 



Southern' Area. 



This area was described in the first report, in which it was shown 

 to be composed of D. bifidus shales which had been invaded by a thick 

 mass of quartz-porpliyry. The shales, well displayed in the cliffs 

 of Forth Llauog and Foel Fawr, are highly fossiliferous, and a large 

 collection of graptolites has been made from them. They contain 

 layers of coarse agglomeratic tuff, and at Foel Fawr pass upwards 

 into thick beds of tuff which are conformably overlain by grey 

 rhyolites. The tuffs and conglomerate on Carn Llundain belong to 

 the same period of eruption. 



The two points of interest, therefore, which were made the object 

 of mapping the island have been successfully solved. It has been 

 found that the so-called Tremadoc Beds are Arenig sediments, and 

 that they do not pass downwards into the Lingula Flags, but are 

 brought against them by a fault ; also that the rocks hitherto 

 regai'ded as pre-Cambrian belong to a period of igneous activity that 

 occurred in Lower Llanvirn, or even later, times. 



It is hoped that the full description of the district will be completed 

 this year, and it is the present intention of Mr. J. Pringle to com- 

 municate the results of his investigations to the Geological Society 

 of London. 



(7) Structural Features of the Coal-fields of Pennsylvania 

 AND their Influence on the Origin of Hard Coal. By 

 Professor E. S. Moore, M.A., Ph.D. 



ri'^HERE are two main coal-fields in Pennsylvania, the Bituminous 

 J[ and the Anthracite. The latter field comprises an area of 

 approximately 480 square miles situated in the highly folded portion 

 of the Appalachian Province, while the former field covers a much 

 larger area with but gently folded strata. Between these two fields 

 are limited areas underlain by semi-anthracite coal in strata which 

 have suffered a medium amount of diastrophism. 



So close is the relationship between intense diastrophism and the 

 development of anthracite coal, that the influence of pressure — 

 combined with conditions favourable for the escape of the volatile 

 constituents from the vegetable matter — seems to be self-evident, 

 although other theories, such as the action of bacteria, etc., have 

 been advanced to account for the origin of anthracite. The structure 

 of some of the anthracite basins is extremely complex, and the coal 

 can often be mined only by special methods, especially where the 

 Mammoth seam readies 60 feet in thickness. 



In the Bituminous field, 'rolls' and 'horsebacks' are common, 

 and investigation has shown that these, usually, nearly parallel the 

 larger mountain structures. 



