452 GEOLOGY 



residual materials which the advancing Proterozoic seas found 

 upon the surface which they transgressed had arisen chiefly by 

 rock decay, and the material arising from the decomposition of the 

 Archean was unlike the formation from which it came . 



(2) Besides working over the decayed rock, the waves doubtless 

 attacked the solid rock wherever exposures were favorable, just as 

 waves here 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 



Fig. 345. Diagrammatic 'section showing relations which existed around 

 Archean lands during the Huronian period. Huronian sediments (Al) 

 are in process of deposition. They are affected by intrusions and ex- 

 trusions of lava, dz, ds, etc. 



class. The sediments of this second class were more or less inti- 

 mately mingled with those which had been prepared in advance 

 by the decomposition of the rock. 



(3) The streams descending from the land must have brought 

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

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

 though a smaller part was doubtless derived by the mechanical 

 action of running water on undecayed rock. Once in the sea, the 

 river detritus was mingled with the sediments acquired in other 

 ways. 



Since some 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 sediments were, on the whole, more silicious than the 

 rock from which they were derived. 



The sorting power of moving water takes account of the physi- 

 cal conditions and properties of the material handled, and not of 



