300 ANNUAL OF SCIENTIFIC DISCOVERY. 



etc.; while nil the carbon, sulphur, and chlorine, in the form of acid gases, 

 mixed with waiery vapor, azote, and a probable excess of oxygen, would 

 form an exceedingly dense atmosphere. When the cooling permitted con- 

 densation, an acid rain would fall upon the heated crust of the earth, decom- 

 posing the silicates, and giving rise to chlorides and sulphates 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 influence of the hot, moist atmos- 

 phere charged with carbonic acid, and the feldspathic silicates arc 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 bicarbon- 

 ate, where, having first precipitated any dissolved sesquioxides, it changes 

 the dissolved lime-salts into bicarbonate, which, precipitated chemically, or 

 separated by organic agencies, gives rise to limestones, the chloride of cal- 

 cium 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 formations of glauconite, are 

 considerations foreign to our present study. 



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

 tive fused mass, mny be generated the silicious, calcareous, and argillaceous 

 rocks which make up the greater part of the earth's crust, and we also un- 

 derstand the source of the salts of the ocean. But the question here arises, 

 whether this primitive crystalline rock, which probabjy 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 offer, in 

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

 gies with the lavas of modern volcanoes, that they are also universally sup- 

 posed 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 geologi- 

 cal period up to the present day, have been from time to time ejected from 

 below. They arc 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, however, offer very great diversities in their com- 

 position, from the highly silicious and feldspathic granites, eurites, and tra- 

 chytes, 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, dieritc, hypcrite, melaphyre, cuphotide, trap, and basalt; in these, 

 lime, magnesia, and iron-oxide arc abundant, and soda prevails over the pot- 

 ash. To account for these differences in the composition of the injected rocks, 

 Phillips, and after him Durocher, suppose the interior fluid mass to have sepa- 

 rated into a denser stratum of the basic silicates, upon which a lighter and 

 more silicious portion floats, like oil upon water; and that these two liquids, 

 occasionally more or less modified by a partial crystallization and eliquation, 

 or by a rcfusion, give rise to the principal varieties of silicious and basic 

 rocks, while from the mingling of the two zones of liquid matter, interme- 

 diate rocks are formed. 



