284 ORIGIN OF CRYSTALLINE ROCKS. [XIII. 



ophiolites, and talcose, chloritic, and epidotic rocks. Inter- 

 mediate varieties resulting from the association of the minerals 

 of this class with those of the first, and also with the materials 

 of non-silicated rocks, such as limestones and dolomites, show 

 an occasional blending of the conditions under which these 

 various types of rocks were formed. 



The distinctions just drawn between the two great divisions 

 of silicated rocks are not confined to stratified deposits, but 

 are equally well marked in eruptive and unstratified masses, 

 among which the first type is represented by trachytes and 

 granites ; and the second, by dolerites and diorites. This fun- 

 damental difference between acidic and basic rocks, as the two 

 classes have been called, finds its expression in the theories of 

 Phillips, Durocher, and Bunsen, who have deduced all silicated 

 rocks from two supposed layers of molten matter within the 

 earth's crust, consisting respectively of acidic and basic mix- 

 tures ; the trachytic and pyroxenic magmas of Bunsen. From 

 these, by a process of partial crystallization and eliquation, or 

 by commingling in various proportions, those eruptive rocks 

 which depart more or less from the normal types are supposed 

 by the theorists of this school to be generated. (Ante, pages 

 3 and 23.) The doctrine that these .eruptive rocks are not 

 derived directly from a hitherto uncongealed nucleus, but are 

 softened and crystallized sediments, in fact, that the whole of 

 the rocks at present known to us have at one time been 

 aqueous 'deposits, has, however, found its advocates. In sup- 

 port of this view, I have endeavored to show that the natural 

 result of forces constantly in operation tends to resolve me- 

 chanical sediments into two classes : the one coarse, sandy, and 

 permeable ; the other fine, clayey, and impervious. The action 

 of infiltrating atmospheric waters on the first and more sili- 

 cious strata will remove from them lime, magnesia, iron-oxide, 

 and soda, leaving behind silica, alumina, and potash, the 

 elements of granitic, gneissic, and trachytic rocks. The finer 

 and more aluminous sediments (including the ruins of the soft 

 and easily abraded silicates of the pyroxene group), resisting 

 the penetration of the water, will, on the contrary, retain their 



