1862.] JTTEES — RIVEE-VALLEYS. 383 



shale before mentioned. These may be called the Lower Limestone 

 Shale ; and they rarely exceed 200 feet in thickness. South of that 

 latitude, however, beds of black argillaceous matter and of grey 

 quartzose grit come in beneath the Lower Limestone Shale, forming 

 the Carboniferous Slate of Sir R. Griffith, with the Coomhola Grits of 

 the Survey, making a group which rapidly thickens towards the 

 south, until it is 5000 or 6000 feet thick. This group there takes 

 the place of the Carboniferous Limestone in the synclinals, the val- 

 leys generally running along its softer upper parts, while the lower, 

 or Coomhola Grit, portion of it forms the ridges, either alone or in 

 conjunction with the Old Red Sandstone. (See Map, PI. XIX.) 



A few instances occur, in the Carboniferous Slate country, of the 

 highest'ground rising over a synclinal curve. Shehy Mountain (1797 

 feet), north of Dunmanway, is the most conspicuous of these ; but a 

 few othea* minor cases occur. 



Former extension of the Upper Palceozoic EocJcs. — It has been stated 

 that, in the^district north of the Blackwater valley, the Carboniferous 

 Limestone forms one continuous sheet, with the exception of those 

 parts where the Lower Palaeozoic rocks or the Old Red Sandstone 

 appear through it. As the beds of the limestone always rise on all 

 sides towards these protruded mounds, and are cut off successively 

 as they approach them, no one can, I think, fail to recognize in these 

 local protrusions the character of accidental holes of erosion in the 

 once continuous sheet, and that the limestone beds formerly stretched 

 horizontally across the areas where these holes occur. In other 

 words, the limestone once spread continuously in horizontal beds over 

 the spaces where we now find the hills of Slieve Bloom and the rest. 



AYhen, moreover, we come to examine the isolated Coal-measure 

 districts which rest on the limestone, and find them all made of pre- 

 cisely similar beds, with similar fossils, and find also that wherever 

 the uppermost bed of the limestone dips beneath the present surface 

 of the groimd, or wherever a hill rises to sufficient elevation above the 

 limestone plain to take in the top bed of the limestone beneath 

 its surface, the lowest beds of the Coal-measures always come in 

 over that bed, with these invariable characters, we are similarly led 

 to the conviction that the Coal-measures were formerly continuous 

 over the whole of the limestone. 



Similar reasoning holds good for the former persistence of the 

 limestone over the district south of the Blackwater Valley, inasmuch 

 as we always find the limestone coming in wherever the uppermost 

 bed of the Old Red Sandstone dips beneath the present surface, so 

 as to allow of the whole of the black shales to sink beneath it also 

 and the lowest bed of the limestone to appear in its natural po- 

 sition. 



Where the Carboniferous Slate comes in with so great a thickness 

 as it does in the south-west of Ireland, it of course precludes the 

 appearance of the limestone, which could only come into the ground in 

 one of two cases — either if that ground had been much loftier than 

 it is, the folds of the rocks remaining the same, or if, the outline of 

 the ground remaining the same, the dip of the beds had been more 



