GORGES AND RAVINES. 171 



after the first filling of the fissure, which has caused 

 a second fissure, now occupied by the minerals. To 

 return, however, to the gorges now in question no 

 matter what was the original cause of the fissures, 

 the present gorges are due to the weathering away of 

 igneous dykes, which happen to be in these cases 

 more easily denuded than the associated rocks. In 

 the west of Galway and Mayo there are some 

 marked passes of this kind which for miles have 

 nearly perfectly level floors ; these are locally called 

 " Bohernacolley " (The hags' road or path), as they 

 are supposed to have been formed by witches. 

 Here the dykes are generally a variety of whinstone 

 which weathers rapidly, principally by chemical 

 action, leaving standing on both sides walls of granite 

 or metamorphic rocks. In other places, however, 

 there are dykes of felstone, which at first sight might 

 be expected to resist denudation better than the 

 associated rocks ; but they do not, on account of in- 

 numerable systems of joints, which break them up 

 into a small angular gravel. Here it should also be 

 mentioned, although not connected with gorges, that 

 some whinstone dykes in that country, although 

 to the eye very similar to those that weather away, 

 have resisted denudation better than the metamorphic 

 and other associated rocks, and now stand out in 

 thick wall-like masses, stretching in more or less 

 regular lines along and over the hills. 



