16 The Ainericfni Geologist July, isfte 
pile of strata was in Pembrokeshire, (Uirdiii,aiisliire, or a little 
west of them, and therefore it was there that subsidence was 
greatest and most heat or maximum debituminization of the 
coal elfected the marked siliceous or sand_y character of the 
measures, aiding, by reason of its included water, the process, 
not a little. That the noted gradual change in the composition 
or kinds of coal of the South Wales basin will be in general 
agreement with the hydrothernial hyi)Othesis now advanced 
seems proba])le. The deep-seated hydrothernial theor}^ for 
anthracite would appear to furnish a reasonable explanation 
as to why, in the South Wales coal field, the lower coals, as 
followed east beneath the upper seams, become more bitumi- 
nous. The theory also provides a reasonable explanation of 
the phenomenon of practically flat beds of pure anthracite as 
they occur in Ireland; a dirtieulty in accounting for which 
has we think hitherto been felt. Structurally' then, there 
is little difference between the anthracite regions of Wales 
and Ireland and those of Pennsylvania; especially when the 
points of the compass have been reversed, because in Penn- 
sylvania the change towards anthracite is in the opposite 
direction to what it is in Wales. However it all points to a 
continent in the North Atlantic region from which the Paleo- 
zoic strata were derived, and which perhaps operated princi- 
pally in producing the great regional upthrust and puckerings 
of the earth-crust after the coal period. 
That the Coal Measures in Devonshire and in Brittany, 
whose leading features may be said to be great thickness of 
sandy shales, sandstones, grit and compact gray quartzites,* 
with interstratitied, though thin and unimportant (commer- 
cially) layers of anthracite, highly crumpled and often much 
altered ( V metamorphosed), and depressed between elevated 
areas of older rocks — is significant in this investigation. 
In passing, we should not forget the great thickness and 
sandstone characteristic of the Somersetshire coal field and 
its suggestive highly-disturbed southern border, along which, 
unless I mistake, the lowest coal beds are less bituminous than 
the higher and flatter seams. 
Sir Arch. Geikie says " Some Lower Silurian shales are 
black from diffused anthracite." 
=^See De la Beche's report on Cornwall and Devon. 
