478 THE JOURNAL OF GEOLOGY. 



the antecedent of disintegration, of decay, but the two are not 

 most efficiently active at the same time. Now their products 

 differ. Rock breaking yields shingle, gravel, coarse sand of 

 mixed mineralogical composition, and no chemical solutions. 

 Rock-decay yields directly no shingle or gravel, but produces 

 sand, chiefly quartz-sand, clay, silt and chemical solutions. 

 Hence, if the products of rock-breaking are deposited unchanged 

 in the sea, there will result one class of sediments from which we 

 may infer corresponding conditions of erosion of the parent land ; 

 and if the products of rock-decay are deposited we must infer 

 other conditions of erosion. 



Declivity is the chief factor which determines either rock- 

 breaking or rock decay. Rock breaking occurs on steep slopes, 

 that is, among hills or mountains ; rock-decay takes place chiefly 

 on gentle slopes, that is, in valleys or on plains. Hence the 

 sediments may indicate the topographic phase of the parent-land. 



They may indicate topographic phase, not permanent topo- 

 graphic character, for relief of the land surface is transient. The 

 steeps of mountains become the slopes of hills, the hill slopes sink 

 to plains and plains to base-level ; and erosion pauses till renewed 

 by uplift. So the conditions of rock-breaking pass into those of 

 rock-decay, and the product of the two processes may appear in 

 sediments, the older gravel and sand beneath the younger sandy 

 clay and clay. 



The possible sequence of unlike sediments does not stop with 

 the finer mechanical products of disintegration ; chemical solu- 

 tions may be related to chemical or organic deposits, and these 

 have their place among strata. The amount of lime and mag- 

 nesia carried annually from a given land area is directly related 

 to the efficiency of rock-decay, and so among other factors to 

 slope. Rock-decay is limited on the one hand by declivities, 

 which promote the rapid running off of rainfall, and on the other 

 hand by the accumulation of a deep covering of soil, which pre- 

 vents percolation. Other things being equal, it is probably most 

 efficient during the period corresponding with the life of low 

 hills and sloping plains. If at any time chemical solutions from 



