8/0 STUDIES FOR STUDENTS 



are those of continual percolation by waters carrying the organic 

 acids already described. It naturally follows therefore that a 

 purely chemical decay will progress more rapidly where the 

 rock-mass is covered by such a layer of vegetable soil as shall 

 give rise to the decomposing solutions. Hence, that such an 

 accumulation having begun decomposition will keep on at an ever 

 increasing rate to a depth concerning which we have at present 

 no data for calculation. It must not be too hastily assumed 

 from this that rocks thus protected do in reality disintegrate 

 more rapidly than those exposed on bare hillsides, since here, 

 where physical causes predominate, the loosened particles are 

 removed as fast as formed, and, besides leaving no measure of 

 the destruction going steadily on, new surfaces for attack are 

 being continually exposed. Moreover, in assuming that rocks 

 decay rapidly where covered by vegetation we must not over- 

 look the fact that the character of the overlying soil may be 

 such as to be protective rather than otherwise. Thus, in glaci- 

 ated regions it is a well-known fact that the stris on rock sur- 

 faces are found best preserved where they have been protected 

 from heat and frost by a mantle of drift, or the compact turf so 

 characteristic of the northern states. 



The principles involved in the decomposition of fragmental 

 and stratified rocks are not so different from those we have been 

 discussing as to call for detailed consideration. It is well to 

 note, however, that the materials composing rocks of this type 

 are themselves a product of these very disintegrating and decom- 

 posing agencies, but which have become consolidated into rock- 

 masses, and now, once more in the infinite cycle of change, are 

 undergoing a breaking up. It follows from the very nature of 

 the case that such rocks, with the exception of the purely cal- 

 careous varieties, will undergo less chemical change than do those 

 we have been discussing. Their feldspathic and easily decom- 

 posable silicate constituents long ago yielded to the decomposing 

 processes, and were largely or in part removed before consolida- 

 tion took place. Thus most sandstones are composed largely of 

 quartzose sand, the least soluble and least changeable product, it 



