THE VALLEYS OF SOME OF THE IRISH LAKES. 145 



beds in the face of a low perpendicular cliff. This 

 cliff margins a turlough, and the usual winter level 

 of its water seems to agree with this line of weather- 

 ing, the note made being, " Cup weathering at the top 

 of the winter floods, but over it are two feet of rock ; 

 these holes may be possibly due to the decay of 

 plants." 1 



Away from lakes a weathering generally takes place 

 in connection with the joint-lines. If the rocks are 

 thin bedded, it breaks them up into a very coarse 

 shingle, but if thick bedded, the weather will open 

 narrow lines of more or less deep fissures. Such 

 fissures, although connected with, are not -the true 

 pot-holes, as for the latter to be formed there must 

 be portions of the rock of a softer consistency than 

 the rest, so that some places in the mass will weather 

 more freely than the rest, in which places the pot- 

 holes form. If due solely to the dissolving action of 

 acid on a homogeneous rock mass, pot-holes ought to 

 be circular. This, however, is rarely the case, as in 

 general they are more or less rudely oblong; their 

 longest axis always coincides with the jointing of the 

 rocks. Further more, the pot-holes usually occur in 

 systems of lines ; and our colleague, Mr W. Topley, 

 F.Gr.S., when writing of the pot-holes in the chalk 

 bounding the Medway valley, says, " The marked 

 parallelism of the long pipes at Maidstone is an argu- 



1 " Memoirs of the Geol. Survey of Ireland," Ex. sheet 95, p. 47. 



K 



