142 THE VALLEYS OF SOME OF THE IRISH LAKES. 



Pot-holes, Pipes, and the like, as all the low-lying 

 lakes in Ireland seem to have now, or to have had at 

 a former period, some connection with them. 



We have already pointed out how a portion of 

 land may sink if an underlying subordinate bed or 

 portion of a bed is subtracted by water. Sometimes 

 these subsidences form bowl-shaped hollows, but at 

 other times a regular hole will form, called a sluggy 

 or swallow-hole in Ireland, and a pot-hole or pipe in 

 other places. Sluggys generally are somewhat like 

 the shape of an hour-glass or of a funnel ; they may, 

 however, have vertical sides. Pipes and pot-holes in 

 limestone and chalk countries are supposed by sub- 

 aerialists to be " produced by the slow dissolving 

 action of water charged with carbonic acid." This 

 view seems to be now generally adopted ; but although 

 in some few cases carbonic acid may act alone, yet 

 it appears to us to act in most cases in conjunction 

 with lines of joints, structure or other weaknesses 

 in the rocks. It is argued that because some small 

 pipes can be proved to have been excavated by dis- 

 solving action, that all large pot-holes are similarly 

 produced ; this, however, seems to be a weak 

 argument. In various places in Ireland holes 

 similar to what are called " pipes" in England are 

 in course of formation among the tracts of carboni- 

 ferous limestone, and from these we learn that such 

 holes may be formed by two different processes one 



