202 ON THE THKORY OF IGNEOUS ROCKS AND VOLCANOS. 



moist atmosphere charged "with carbonic acid, and the feldspathic sili- 

 cates 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 sesquioxyds, it changes the dissolved lime-salts into bicar- 

 bonate, which precipitated chemically or separated by organic agencies, 

 gives rise to limestones, the chlorid of calcium being at the same time 

 replaced by common salt. The separation from the water 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 present 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 

 questioii here arises whether this primitive crystalline rock, which pro- 

 bably 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. These ofPer, in their 

 mode of occurrence, not less than their composition, so many analo- 

 gies with the lavas of modern volcanos, that they are also 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 to time ejected from below. They are generally regarded as 

 evidences both of the igneous fusion of the interior of our planet, and 

 of a direct communication between the surface and the fluid nucleus, 

 which is supposed to be the source of the various ejected rocks. 



These intrusive masses, howevei', offer very great diversities in their 

 composition, from the highly silicious and felspathic granites, eurites,. 

 and trachytes, in which lime, magnesia and iron are present in very 

 small quantities, and in which potash is the predominant alkali, to 

 those denser basic rocks, dolorite, dierite, hyperite, melaphyre, eupho- 

 tide, trap and basalt; in these, lime, magnesia and iron-oxyd are 

 abundant, and soda prevails over the potash. To account for these 

 differences in the composition of the injected, rocks, Phillips,, and after 

 him Durocher, suppose the interior fluid mass to have separated into a 



