THE VALLEYS OF SOME OF THE IRISH LAKES. 147 



quarrying or other works, thick deposits of clay 

 occur, evidently the insoluble residue of the lime- 

 stone. As this clay is exactly similar to the clay 

 lining the pot-holes, it seems to suggest that both 

 were due to the same cause. 



The swallow-holes, formed from below, mark the 

 lines of subterranean streams. In company with 

 our colleague, the late Mr F. J. Foot, M.A., &c., we 

 explored many of these passages, they being of 

 frequent occurrence in the Counties Clare and Gal- 

 way, and in all places found that they were induced 

 by joints and other breaks in the strata along 

 which the water worked, gradually widening them, 

 partly by dissolving away the rock ; but after the 

 passage had become large, and the stream had 

 increased in size from the surface drainage being 

 diverted, the current acted more like an overground 

 one, abrading as well as dissolving away the rocks. 

 In underground rivers and streams during flood, 

 when the passages are choked with water, the latter 

 acts upwards in jointed parts and other weak 

 portions of the roof, till eventually an opening 

 connected with the surface is formed, in consequence 

 of a portion of the superincumbent mass giving 

 way and falling in, while afterwards the holes are 

 gradually increased in size by meteoric abrasion. 

 Pot or swallow holes in all stages of progress may 

 be seen in the plains of Gralway and Mayo ; the rock 



