THE CARBONIFEROUS SYSTEM 269 



Edge Coal Group. This is a shale and sandstone series, and 

 it includes some valuable seams of coal, ironstone, and oil-shale, 

 1ut no limestones. In Midlothian its thickness is about 800 feet. 

 Some of the beds contain marine shells, others are full of terrestrial 

 ]>l,mts, showing that the gradual submergence which was in progress 

 was now and then counterbalanced by the deposition of the sedi- 

 ment brought down in such quantity by rivers. 



Upper Limestone Group. This is about 600 feet in 

 Midlothian ; it contains three bands of limestone and four seams 

 of coal, interstratified with, sandstone, shale, and fireclay. The 

 limestones contain the same fossils as those of the lower group, 

 and though seldom more than three or four feet thick have been 

 traced over an area of at least 1000 square miles, thus proving 

 that the physical conditions of the period were very uniform over 

 a large area. 



In the western part of the area (Glasgow, Renfrew, and Arran) 

 all these equivalents of the Yoredale Beds are much thinner, the 

 Lower limestones being only 60 feet, the Edge Coal Group about 

 200, and the Upper limestones about 150 feet. All of them are 

 found in Arran, where the red Corrie limestone is regarded as the 

 equivalent of the Hurlet limestone of the mainland, for many of 

 the beds are stained red in Arran. 



7. Ireland 



In no country can the rocks of the Carboniferous System be 

 better studied than in Ireland. They occupy fully one-half of the 

 superficial area of the country ; the whole of the great central plain, 

 which stretches completely across the island from east 'to west, 

 consisting of Carboniferous limestones, though its surface is often 

 covered by Pleistocene gravels (see map, Fig. 94). Here and there 

 this limestone plain supports hills of low elevation composed of 

 the Coal-measures, these outliers being the sole remnants of the 

 beds which once spread over the limestone; their survival being 

 due to the circumstance of their lying in synclinal basins. 



For the sake of clearness and brevity the chief facts regarding 

 the Avonian Series of Ireland will be given under three heads, 

 (1) Northern counties, (2) The Great Central Plain, (3) Southern 

 counties ; the first and third of these areas each exhibiting a special 

 and peculiar facies of the formation differing greatly from that of 

 the central area. 



Northern Counties. The Carboniferous rocks of the more 

 northern counties are merely an extension of the Scotch facie.*, 

 but show a passage into the limestone facies as they are traced into 



