214 The Irish Naturalist, vSeptember, 



DIA TO MA CE^, 



BY W. A. FIRTH. 



Mr. Welch has seut me some specimens of the molhisc Planorbis crista and 

 its var. nautikiis encrusted with diatoms, dredged in a pond near Kilma- 

 cowan old church. I have gone over them very carefull3^ There is an 

 abundance of Cocconeis placentula. Also, sparingly, two Naviciilce., which 

 I believe to be N. radiosa and N. ambigua, but as they are very minute, 

 I cannot be quite positive of these species. Two other species are 

 present — Epithemia gibba and Nitzschia dubia, 



Belfast. 



I v.— G E O I. O G Y. 



BY PROF. G. A. J. COI.B, F.G.S., M.R.I.A. 



The region in which Sligo stands is remarkable for its bold scarps of 

 rock, separating high plateaux from slopes of cultivated land. The 

 Carboniferous limestone series, worn down and depressed in the great 

 central plain of Ireland, is here elevated in the west, and outliers of its 

 lowest strata are found 2,000 feet up on the mountain crests of Conne- 

 mara and Donegal. Whether we approach Sligo by Boyle or by Ennis- 

 killen, we find ourselves in a t3'pically Carboniferous country, where the 

 limestone graduates upwards into the sandstones and shales of the 

 overlying Millstone Grit and Coal Measures. The lower beds are often 

 more yielding than the upper, and become grass-grown, showing occa- 

 sional grey taluses from the massive rocks above. The higher limestone, 

 with its uniform structure and characteristic vertical jointing, weathers 

 out in superb scarps that remind us of those of the North of England. The 

 approximately horizontal position of the strata gives rise to conspicuous 

 plateau-structure (Plate 13). This is nowhere better seen than from the 

 road just south of Sligo town, where the great inland cliff of Ben Bulben 

 (1.722ft.) seems to rise immediately behind the city, and is repeated in 

 the outlying mass of Knocknarea upon the south. The terraced hills 

 north of Lough Gill continue the same scenic features, with woods on 

 the sheltered land below, and short upland grass among the grey steps 

 that form the heights. On the south side of the lake, however, the hills 



