2 THEORY OF IGNEOUS ROCKS AND VOLCANOES. [I. 



phates of the various bases, while the separated silica would 

 probably take the form of crystalline quartz. 



In the next stage, the portions of the primitive crust not 

 covered by the ocean undergo a decomposition under the influ- 

 ence of the hot moist atmosphere charged with carbonic acid, 

 and the feldspathic silicates are converted into clays with 

 separation of an alkaline silicate, which, decomposed by the 

 carbonic acid, finds its way to the sea in the form of alkaline 

 bicarbonate, where, having first precipitated any dissolved ses- 

 quioxides, it changes the dissolved lime-salts into bicarbonate. 

 This, precipitated chemically or separated by organic agencies, 

 gives rise to limestones, the chloride of calcium being at the 

 same time replaced by common salt.* The separation from 

 the waters of the ocean of gypsum and sea-salt, and of the 

 salts of potash by the agency of marine plants, and by the 

 formation of glauconite, are considerations foreign to our pres- 

 ent study. 



In this way we obtain a notion of the processes by which, 

 from a primitive fused mass, may be generated the silicious, 

 calcareous, and argillaceous rocks which make up the greater 

 part of the earth's crust, and we also understand the source of 

 the salts of the ocean. But the question here arises whether 

 this primitive crystalline rock, which probably approached to 

 dolerite in its composition, is now anywhere visible upon the 

 earth's surface. It is certain that the oldest known rocks are 

 stratified deposits of limestone, clay, and sands, generally in a 

 highly altered condition, but these, as well as more recent 

 strata, are penetrated by various injected rocks, such as granites, 

 trachytes, syenites, porphyries, dolerites, phonolites, etc. i 

 offer in their mode of occurrence, not less than their compo- 

 sition, so many analogies with the lavas of modern volcanoes, 

 that they also are universally supposed to be of igneous origin, 

 and to owe their peculiarities to slow cooling under pressure. 

 This conclusion being admitted, we proceed to inquire into the 

 sources of these liquid masses which, from the earliest known 

 geological period up to the present day, have been from time 

 * See in this connection the note appended, .page 10. 



