SILICEOUS ROCKS 2C>5 



nature, is usually imperfect, and changes from point to point, so 

 that the sedimentary rocks have an even less definite chemical 

 composition than have the igneous. 



The most useful classification of the sedimentary rocks is, pri- 

 marily, according to the mode of their formation, and secondarily, 

 according to their composition. This gives two principal divi- 

 sions : I, the Aqueous Rocks, or those laid down under water ; 

 II, the s&olian Rocks, those which were accumulated on land, 

 which are of very limited extent and importance. 



The aqueous rocks may be further divided into three classes : 

 1, Mechanical Deposits; 2, Chemical Precipitates; 3, Organic 

 Accumulations. 



I. Aqueous Rocks 



The rocks laid down under water form by far the largest and 

 most important of the sedimentary series. 



I. MECHANICAL DEPOSITS 



These have resulted from the accumulation of debris derived 

 from the destruction of preexisting rocks, carried in mechanical 

 suspension by moving water, whether waves, currents, or streams, 

 and dropped when the velocity of the moving water was no longer 

 sufficient to carry them. The study of the dynamical processes 

 has already taught us that such accumulations are forming to-day 

 in all kinds of bodies of water, and an examination of the rocks 

 will show that similar accumulations have been made since the 

 beginning of recorded geological time. Mineralogically, the 

 mechanical deposits are of two principal kinds, the siliceous and 

 the argillaceous. The sorting power of water has been sufficient 

 to separate them roughly, though we find mixtures of the two in 

 all proportions. 



a. Siliceous Rocks 



In these rocks the principal component is quartz in fragments 

 of greater or less size, either angular, or more or less rounded by 

 wear. Of the common rock-forming minerals quartz is the hardest 



