156 THE VALLEYS OF SOME OF THE IRISH LAKES. 



different bays. We have not, however, as carefully 

 examined any of this ground, except the valley of 

 Youghal Bay, in which a fault evidently exists. This 

 lake is one of the natural reservoirs on the Shannon, 

 and at certain seasons the water is very muddy, 

 while at other times there is a great evaporation from 

 it, which causes large deposits of mud and marl ; 

 therefore from the soundings very little can be 

 learned as to the nature of the original bottom of the 

 basin, in addition to the two facts of the deeps being 

 in the lines of the break, or at the junction of two 

 or more, and of the islands, rocks, and shoals being 

 on the upthrow sides of the faults. This latter fact is 

 very apparent, except in the great east and west reach, 

 on account of the throws of the south faults of the 

 Scarriff valley being undecided. 



We have next to consider how the rock-basin of 

 Lough Derg was excavated, for although the water 

 is now, in places, edged with drift or alluvium, yet 

 formerly the area was nearly, if not quite, surrounded 

 by rocks. The sea probably was the first excavator, 

 marking out, along the faults and breaks, valleys and 

 ravines afterward to be occupied by ice and rivers, 

 but when it retired, the other denudants did more 

 or less work. Principal Dawson, LL.D., M f Gill's 

 College, Montreal, has pointed out, in relation to 

 some of the large North American lakes, that 

 apparently the original surface passages from them 



