168 GEOLOGY. 



and there cut into solid rock at the present time. The materials thus 

 derived resembled the parent formation in average composition, and 

 are thus distinguished from those of the preceding class. The sedi- 

 ments of this second class were more or less intimately mingled with 

 those which had been prepared in advance by the decomposition of the 

 rock. 



(3) The streams descending from the land brought down their 

 quota of gravel, sand, and mud. The larger part of the river-borne 

 detritus was probably made up of the decomposition products of rock, 

 though a smaller part was probably derived by the mechanical action 

 of running water on undecomposed terranes. Once in the sea, the 

 detritus which the rivers brought down from the land was mingled and 

 deposited with the sediments acquired in other ways. Then, as now, 

 minor quantities of terrigenous sediment were doubtless blown from 

 the land to the sea, and added to that derived from the land in other 

 ways. 



Since much of the more soluble constituents of the Archean rock 

 extracted during the processes of decomposition probably remained 

 in solution in the sea-water, it is to be presumed that the clastic sedi- 

 ments were, on the whole, more siliceous than the rock from which they 

 were derived. 



The sorting power of moving water takes account of the physical 

 conditions and properties of the materials handled, and not of their 

 chemical constitution; but in the decomposition of Archean rock, 

 the quartz remaining in the residual mantle was generally in larger 

 particles than the aluminous products of the decomposition of the 

 silicates, and under the assorting influence of the waves the quartz 

 grains (sand) were more or less completely separated from the alumi- 

 nous particles (mud). Thus materials which were unlike chemically 

 were separated from one another because they were unlike physically. 



If the Proterozoic seas harbored abundant shell-bearing life, or 

 if their waters anywhere became overcharged with lime carbonate in 

 solution, limestone might have been formed, and should be found 

 associated with the other sedimentary strata. 



Extent. — While the beds of sediment accumulated in the Protero- 

 zoic are known in limited areas only, it is to be borne in mind that the 

 Proterozoic sediments were in reality as widespread as the Proterozoic 

 seas; for though coarse material from the land is not usually washed 



