WEATHEBIKG- OP GBAJtflTE 187 



the residual sand, yielded the results given in the columns cor- 

 respondingly numbered on the preceding page. 



In glancing over these figures it is at once apparent that 

 there is a surprisingly small difference in ultimate composition 

 between the sound rock and the residual sand, the more marked 

 differences being a slightly smaller amount of silica, more alu- 

 mina, and slightly diminished amounts of lime, magnesia, pot- 

 ash, and soda, with a considerable increase in the amount of 

 water. The feirous salts have moreover been converted into 

 ferric forms. It does not necessarily follow, however, that no 

 more actual gain or loss of material or change in manner of 

 combination than is here indicated may not have taken place, 

 and at the very outset it may be well to enter into a discussion 

 of the manner in which the results of such analyses are to be 

 considered. 



We must first of all remember that any indicated loss or 

 gain of a constituent may be only apparent, and that the true 

 relative proportions can be learned only by calculating results 

 of analyses of both fresh and decomposed materials on a com- 

 mon basis Thus the first glance at analysis III, as given, 

 might lead one to surmise that the decomposed rock had actually 

 lost only some 3.64% of silica. This, however, is not strictly 

 the case, since this analysis shows 4.7% volatile constituents 

 against 1.22% in analysis I of the fresh material. Could we 

 assume that this difference of 3.48% was due wholly to a 

 uniform absorption of moisture, as by a clay, the problem would 

 resolve itself into simply recalculating all analyses upon a 

 water-free basis. 



The results obtained thus are not quite satisfactory, however, 

 and it is thought a more correct view of the changes taking 

 place may be obtained by assuming for one of the constituents 

 a fairly constant value and using this as a basis for comparison. 



Of all the essential constituents occurring in appreciable 

 quantities in siliceous crystalline rocks the alumina and the iron 

 oxides are the most refractory and the least liable to be removed 

 by a leaching process, although they may undergo manifold 

 changes in mode of combination. Although not absolutely 

 correct, therefore, we will for our present purposes assume the 

 one 01 the other of these (in this case the iron as FcaOg) as a 

 constant factor, and in order to show the proportional or actual 

 amount of loss of any constituent will recalculate the analyses 



