878 A TREATISE ON METAMOKPH1SM. 



and have been alluded to in this treatise. (See Chapter VI, pp. 496-497.) 

 Nothing further will he said upon their manner of formation. The essential 

 point in this connection is that such sands are likely to be not at all, or 

 very slightly, sorted; therefore they contain practically all the minerals 

 of the original rocks from which they are derived, and hence include 

 the ferromagnesian minerals. While such formations are of very great 

 thickness in various parts of the world, and doubtless are now being 

 indurated in their lower parts, as described below, it is difficult to ascertain 

 how far the ferromagnesian sands, and their altered equivalents thus 

 formed, have been preserved among the rocks of past geological ages. 



Ferromagnesian sands are being extensively deposited below the waters 

 of inland seas and of the ocean, where the sorting is very imperfect. This 

 is likely to be the case in the estuaries or near the mouths of strong rivers 

 which are rapid to their mouths. All these conditions are well illustrated 

 by some of the rivers which flow into the estuaries of the Atlantic, such as 

 the Susquehanna. The conditions for the deposit of ferromagnesian sands 

 are still more nearly perfect along the Pacific coast. The rapid streams of 

 the Coast Range carry the extremely varied material of the mountains to 

 the bays, such as San Diego, San Francisco, and Puget Sound, or to the 

 ocean. But in the open ocean the ferromagnesian sands are likely to be 

 sorted into their constituent minerals. 



While the perfect conditions for the formation of the ferromagnesian 

 sands are those of disintegration with no decomposition, of course decompo- 

 sition does everywhere occur to a greater or less degree. In the case of the 

 rivers of the Piedmont plateau, which are emptying into estuaries, the 

 decomposition of the sands is considerably advanced. But the decomposi- 

 tion of the sands of the Coast Range is not nearly so far advanced. 



Since decomposition everywhere occurs to some extent, the ferromag- 

 nesian minerals and the feldspars have been changed more or less to 

 minerals into which they commonly alter in the belts of weathering and 

 cementation. The feldspars may have been changed in part to kaolin, mica, 

 quartz, zeolites, etc. The pyroxenes, amphiboles, and biotite may have 

 been changed to a greater or less extent to chlorite, epidote, serpentine, 

 talc, etc. Thus in the sands under consideration there may be present not 

 only original minerals, but any of their alteration products in various 

 proportions. In so far as alterations have taken place the proportion of the 



