382 TRANSACTIONS OF SECTION C. 
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 
—_-zeems to be self-evident, although other theories, such as the action of bacteria, 
&c., 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 reaches 
60 feet in thickness. 
Tn the Bituminous field, ‘rolls’ and ‘ horsebacks’ are common, and investiga- 
tion has shown that these are, usually, nearly parallel to the larger mountain 
structures. 
