138 PROCEEDINGS OF SOCIETIES. 



800 feet in thickness, under which, for the first time in this line of section, 

 we have a considerable development of gray sandstone, which in this loca- 

 lity extends down to the sea-shore, and beneath which, to the northward, 

 there is a considerable thickness of beds, consisting of alternations of calp 

 and shale, similar to those which lie above it. These strata are well 

 exposed to view on the west coast of the counties of Sligo and Leitrim, 

 extending northward for upwards of sixteen miles from Glencar, by Ben- 

 bulben, to Dartree mountain, near Bundoran ; thence for fifteen miles, 

 in an eastern direction, by Garrison and Belleek, to Lower Lough Erne, 

 and afterwards in a southern direction, for fourteen miles, from Churchill, 

 by Derrygonnelly and Belmore mountain, to the Arney river, near Flo- 

 rence-court, where it gradually thins out. And thus we have a total 

 length of thirty-five miles, in a northern, eastern, and southern direc- 

 tion, in which these remarkable sandstone and shale beds are fully 

 exposed for examination to the geological observer. 



In the line of section engraved on the western margin of the " Geo- 

 logical Map," which passes over the summit of Dartree mountain, at an 

 elevation of 1 712 feet above the level of the sea, and which extends north- 

 ward to Ballyshannon, &c, we find that the Upper or splintery Limestone 

 gives a thickness of 500 feet ; the Upper Calp Shale beneath it, 500 

 feet ; the subjacent sandstone beds, 800 feet ; the Lower Calp Shale, 

 500 feet; and the Lower Limestone, extending northward from Bun- 

 doran to Ballyshannon, 600 feet, making a total thickness in this 

 locality of 2900 feet, of which 1800 feet belong to the Calp series. But 

 this is an unusual development, as in other cases the thickness does 

 not exceed half that amount, the variation in this respect being caused 

 by the gradual thinning out of one or more of the members, particularly 

 of the sandstone, which at the opening of Glencar, at the southern base 

 of Benbulben, commences in the form of a wedge, having shale and 

 impure limestone beds both above and below, and gradually thickens as 

 it extends northward ; and a similar occurrence takes place near the 

 southern termination of the Arney River. Thus the eastern escarpment 

 of Glenkeel mountain, south of Derrygonnelly, and ten miles north of the 

 Arney River, gives nearly a similar section to that of Dartree mountain, 

 presenting the Upper Limestone, the Upper Calp, the Sandstone beds, the 

 Lower Calp, and subjacent Lower Limestone at Ely Lodge, on Lough Erne. 

 Here the Sandstone beds are of considerable thickness ; but to the south- 

 ward they gradually diminish, and, after passing the Arney River are 

 no longer visible, having thinned out, as at Glencar, and are lost in the 

 union of the Upper and Lower Calp series; and these united beds continue 

 thence through Elorence-court to the valley of Swanlinbar, in which no 

 beds of sandstone have been observed. 



The variable thickness of the sandstone beds and their general fugi- 

 tive character are familiar to geological observers ; but the facts here 

 related afford a remarkable example. 



On the margin of the last edition of my large " Geological Map of 

 Ireland" I have given a section which exhibits the entire suite of the 

 Carboniferous System of Ireland, and which passes through the larger 



