Evans. — On Contact Metamorphism. 561 



dolerite intervenes between the coal-measures and the new 

 red sandstone. Ac Whitwick Colhery it is 60 ft. thick, and 

 has turned to cinders a seam of coal with which it comes in 

 contact. 



" The Cornbrook Coalfield is to a large extent covered by 

 basalt, from 60 ft. to 150 ft. thick, and in some places the coal 

 is altered and ' sooty.' The known instances of this class are 

 well summed up by Geikie in his ' Outlines of Field Geology.' "* 

 " Sometimes," he says, " the coal has been entirely consumed, 

 and a layer of igneous rock has taken its place. At other 

 times a thin sheet of molten lava has been injected along the 

 top, bottom, or centre of the coal-seam, converting it into a 

 kind of anthracite or into a mere cinder. Examples may be 

 found where the coal has been fused into a cellular mass, and 

 has subsequently had its vesicles filled- with infiltrated car- 

 bonate of lime. In Ayrshire numerous beautiful sections 

 have been laid bare, when the coal has been rendered pris- 

 matic, the hexagonal or polygonal prisms, like so many 

 bundles of pencils, diverging from the surface of the intruded 

 igneous rock." 



To the second, or " pressure," class belongs the great 

 anthracite bed of the Appalachian system. " In Pennsylvania 

 the strata of coal are horizontal to the westward of the 

 Appalachian Mountains, where Professor Eogers pointed out 

 that they were most bituminous ; but as we travel south- 

 eastward, where they no longer remain level and unbroken, 

 the same seams become progressively debituminized in pro- 

 portion as the rocks become more bent and distorted. At first 

 on the Ohio Eiver the proportion of hydrogen, oxygen, and 

 other volatile matters ranges from 40 to 50 per cent. East- 

 ward of this line, on the Monongahela, it still approaches 

 40 per cent., when the strata begin to experience some gentle 

 flexures. On entering the Appalachian Mouatains, where the 

 distinct anticlinal axes begin to show themselves, but before 

 the dislocations are considerable, the volatile matter is gene- 

 rally in proportion of 18 or 20 per cent. At length, when we 

 arrive at some isolated coalfields associated with the boldest 

 flexures of the Appalachian chain, where the strata have been 

 actually turned over — as near Pottsville — we find the coal to 

 contain only from 6 per cent, of volatile matter, thus becom- 

 ing a genuine anthracite."! Portions of the Pembrokeshire 

 anthracite beds of the South Wales Coalfield belong to this 

 class. 



To the third class belongs much of the South Wales 

 anthracite. Speaking of the gradual change in character of 



* " Outlines of Field Geology," by Sir A. Geikie, p. 160. 

 t Judd's "Lyell,"page33. 

 36 



