G. H. liinahan — ^Yater Basin of Lough Derg, 487 



has shown (Geol. Mag., 1871, Vol. VIII. p. 359, PI. X.) that 

 neither water nor ice alone could have formed the bed of the Lake 

 of Como. In this paper I propose to describe the form of the water 

 basin of Lough Derg, with the features of the principal portions of 

 the adjoining country, and to put forward suggestions as to how the 

 lake basin could have been formed. 



Country about Lough Deeg. , 



Lough Derg is one of the large sheets of water through which 

 the River Shannon flows. Formerly its dimensions must liave been 

 considerably larger ; now, however, they are curtailed in places by 

 accumulations of alluvium and bog, especially towards the north- 

 east. The larger division of the lake basin, extending from its 

 northern extremity at Portumna to the long east and west reach 

 between Youghal and Scarriff bays, is in Carboniferous Limestone ; 

 while a smaller division, forming the south-west arm of the lake 

 near Killaloe, is in slates and grits of Silurian age. The country 

 on the east of the larger division cannot be satisfactorily investi- 

 gated, as the rocks are all so similar, and in places so obscured by 

 drift or bog, that it is nearly impossible to prove faults ; however, it 

 is evident that every bay or creek at the eastern side of the lake 

 lies on a line of break. Youghal Bay, farther south, is known to 

 occupy a line of fault, while in the country on the west of the lake 

 various large faults were proved while working the geology. On 

 this side of the Shannon are two groups of hills, called respectively 

 Slieve Bernagh and Slieve Aughta. The northern portion of the 

 first lies west of the smaller division, on the south-west arm of 

 Lough Derg, and the latter lies west of the larger division, the two 

 groups being separated from one another by the Scarriff Valley. 



In the northern portion of Slieve Bernagh no great faults can be 

 positively proved, although it is probable there are at least two ; but 

 in connexion with Slieve Aughta six nearly east and west faults 

 have been proved by the Government Survey, each associated with 

 a valley or marked feature. They may be called, beginning towards 

 the south, 1st. The Scarriff valley north fault, 2nd. The Cloonnagro 

 and Corra valley fault, 3rd. The Lough Atorich valley fault, 4:th. The 

 Derrybrian valley fault, 5th. The Owennaglanna and Boleyneendorrish 

 valley faidt, and 6th. The Dalystown river valley fault. These faults, 

 from 1 to 4 inclusive, are downthrows to the southward, while 

 numbers 5 and 6 have downthrows to the northward. All of them 

 are of Post-Carboniferous age, and they, as well as others farther 

 south, probably belong to one system, and were formed simul- 

 taneously. The relations between them and the features of the 

 ground were carefully studied during the progress of the Geological 

 Survey, as will appear from the following epitome of the description 

 in the Survey Memoirs. 



In Slieve Aughta there is only one large north and south A'alley, 

 that of Lough Graney ; but the east and west valleys are numerous, 

 and lines of faults have been proved in each. These latter valleys 

 can be traced across the mountains from the limestone country on 



