384 PEocEEDiNGS OF THE GEOLoaiCAL SOCIETY. [June 18, 



steadily persistent in one direction, so as to bring the limestone 

 down beneath its surface. There is no reason to suppose that the 

 limestone or Coal-measures would not have appeared if either of 

 those two circumstances had occurred. 



I have, indeed, as the result of ten years' examination and 

 reflection, arrived at the full belief that, wherever in the South^of 

 Ireland we now find Old Red Sandstone, the Carboniferous Limestone 

 and Coal-measures once existed over it — and not only so, but that 

 the upper rocks once spread far beyond the Hmits of the lower. I 

 am, in fact, unable to escape the conviction that at the close of the 

 Carboniferous Period one great plain of Coal-measures extended hori- 

 zontally over all Ireland, with the exception perhaps of the loftier 

 peaks of Connemara, Donegal, Down, and Wicklow, even if any parts 

 of those mountains remained uncovered by the highest Coal-measure 

 beds. 



It is also quite clear that, from the base of the Old Eed Sandstone 

 to the highest bed of the Coal-measures, all the Upper Palseozoic 

 rocks were originally horizontal, and that at the end of the Coal- 

 measure Period they were aU under water. 



It can be shown that aU this vast series of beds was deposited on 

 the slowly subsiding and rather irregular surface of a previously 

 existing land, made of the Lower Palaeozoic rocks, and that the de- 

 pression commenced first on the south or south-west, and continued 

 there for a long time, during the deposition of the great mass of the 

 Old Red Sandstone, before it began to affect the centre of Ireland, 

 where the Old Red Sandstone is comparatively thin or does not exist 

 at all. It then went on again during the deposition of the Coomhola 

 Grits and the chief part of the Carboniferous Slate, without much 

 affecting any part north of the latitude of Cork. It was not until after 

 the deposition of the Carboniferous Slate that the depression became 

 more general, so as to allow of the regular deposition of the Lower 

 Limestone Shale and Carboniferous Limestone. 



The partial nature of the earlier deposits, of course, necessitates a 

 want of strict, parallelism between their beds and those which spread 

 over and beyond them. The departure from strict parallelism, how- 

 ever, would be too slight to be perceptible. In the wedge-shaped 

 mass of the Carboniferous Slate, for instance, the angle included 

 between the planes of the uppermost and lowermost beds would be less 

 than 5°, since the distance between two planes inclined to each other 

 at 5° will in the course of 18 miles exceed 8000 feet. (See Expla- 

 nation to sheet 194, &c., of the Maps of the Geological Survey of 

 Ireland.) 



Part II. — ^The Existing Biveh-yaheys oe the Sotris oe Ieeianb. 



Having thus given a sketch of the form and structure of the 

 country at the present day, and stated generally what they must 

 have been towards the close of the Carboniferous Period, I now 

 proceed to examine the relation between some of the chief river- 

 vaUeys and the subjacent rocks. 



