224 FOKMATION AND GROWTH OF 



position ; therefore on rocks of the first class, soil 

 will form much more rapidly than on those of the 

 second. Some rocks, however, may weather easily 

 into a meteoric accumulation, and yet, on account of 

 a want of a necessary combination of substances, will 

 be incapable of forming a productive soil. Some 

 granytes and sandstones thus weather into " cold 

 soils ; " but if the former be associated with limy 

 matter, and the latter with marly or clayey stuff, a 

 u warm " or productive soil is the result. Other 

 rocks are so chemically formed, that as they 

 weather nearly the whole of their constituents will 

 be carried off in solution. Thus in some parts of the 

 County Galway, over the . carboniferous limestone all 

 the limy matter has been carried away through 

 swallow holes or "sluggys," leaving a siliceous sur- 

 face accumulation, due to the decomposition of the 

 contained chert. In some low-lying places this 

 sandy drift may have been carried down from the 

 adjoining high land ; but when we find it occupying 

 high land, it must have been formed where we find 

 it. 1 Many earthy limestones weather into valuable 

 soils. This, however, depends very much on the 

 nature of the climate ; for while in a moist, cool 

 country the land over limestone may be most fertile, 

 in a dry, hot country the land on an exactly similar 

 rock may be unproductive. 



" Memoirs of the Geol. Survey, Ireland," Ex. sheet, 107, p. 34. 



