Igneous Rocks and Volcanos. 195 



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

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

 and argilaceous 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. These offer, in their mode of occurence, 

 not less than their composition, so many analogies 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 re- 

 garded 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 composition, from the highly silicious and feldspathic granites, 

 eurites, and trachytes, in which lime, magnesia and iron are pre- 

 sent in very small quantities, and in which potash is the predom- 

 inant alkali, to those denser basic rocks, dolerite, diorite, hyperite, 

 melaphyre, euphotide, 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 himDurocher, suppose the interior fluid 

 mass to have separated 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 le?s 

 modified by a partial crystalization and eliquation, or by a refu- 

 sion, give rise to the principal varieties of silicious and basic 

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

 intermediate rocks are formed. (Phillip's Manual of Geology, p. 

 55G, and Durocher, Annates des Mines, 1857, vol. 1, p. 217. 



An analogous view was suggested by Bunsen in his researches 



