152 THE VALLEYS OF SOME OF THE IRISH LAKES. 



deep of the lake lies near the western shore, and has 

 the same bearing as the lake itself, the main joints, 

 and also the ice-striae ; from which it would appear 

 that ice, taking advantage of the joint-lines, was the 

 main excavator of the basin, more especially as all the 

 deeps, except in one place, are more or less gradual 

 and regular in outline, and might have been cut by 

 an agent going from the S.S.W. toward the N.N.E. 

 In this deep place there are no data by which a con- 

 nection between it and the breaks in the country on 

 the west can be traced ; but we know that the country 

 to the N.W. of it is very much cut up with faults, 

 while the rocks are jumped and shifted; one large 

 fault, a downthrow to the N.E., striking for the 

 S.W. end of the great deep (over 150 feet), the deep 

 having a transverse widening in its line of bearing. 

 The bays forming the S.W. arms of Lough Mask 

 are each known to occur along a line of fault, while 

 we learn from the chart that the different shallows 

 and deeps occur respectively on the up- and down- 

 throw sides of the faults, the changes from a shallow 

 to a deep hole being quite sudden. Lough Mask 

 drains by subterranean passages through the barrier 

 of limestone that on the south-east divides it from 

 Lough Corrib, and in the deep just mentioned there 

 is a hole remarkable for being on nearly the same 

 level as the deepest hole in Lough Corrib, the former 

 being 127 feet lower than the Ordnance datum line, 



