87 



a break in the strata coincide. Many other striking 

 examples could be pointed out, especially among the 

 ice-dressed hills of Galway, Kerry, and Cork, 

 where streams have run over polished, scratched, and 

 etched surfaces of rock for ages without having been 

 able to obliterate the ice-marks. 



It has been shown already how rain forms, in the 

 west of Ireland, a protecting envelope of peat which 

 stops denudation ; and not only there, but also over a 

 large portion of Ireland, the moisture of the atmosphere 

 has a very similar effect; a considerable portion of 

 the country having a greater or less coating of peat. 



Chemical action, although an assistant to the other 

 destroyers, may also be, in an indirect way, a preserver, 

 as it disintegrates rocks and forms soil in which plants 

 grow and eventually form a protective envelope. 



We have now given short general descriptions of 

 the work capable of being done by the different 

 denudants, and have pointed out that each, indivi- 

 dually, can accomplish very little ; but if two or more 

 are combined the work may go on much more rapidly, 

 while all work most efficiently when assisted by 

 joints, cracks, and other shrinkage fissures that 

 facilitate the disintegration and quarrying of rock 

 masses. Hereafter, when describing the various 

 features of the earth's surface, we will have to refer 

 to these different denudants, and occasionally may 

 have to describe their work more minutely. 



