206 ON THE THEORY OF IGNEOUS ROCKS AND VOLCANOS. 



that the sihcates of alumina in sedimentary rocks may combine with 

 •alkaline silicates to form feldspars and mica, and that it would be pos- 

 sible to crystallize these minerals from hot alkaline solutions in sealed 

 tubes. In this way I explained the occurrence of these silicates in 

 altered fossilferous strata. My conjectures are now confirmed by the 

 experiments of Daubree, which serve to complete the demonstration 

 of my theory of the normal metamorphism of sedimentary rocks by 

 the interposition of heated alkaline solutions. 



But to return to the question of intrusive rocks : Calculations 

 based on the increasing temperature of the earth's cr4ist as we descend, 

 lead to the belief that at a depth of about twenty-five miles the heat 

 must be sufficient for the igneous fusion of basalt. The recent ob- 

 servations of Hopkins, however, show that the melting points of various 

 bodies, such as wax, sulphur and resin are greatly and progressively 

 raised by pressure, so that from analogy we may conclude that the 

 interior portions of the earth are, although ignited, solid from great 

 pressure. This conclusion accords with the mathematical deductions 

 of Mr. Hopkins, who, fram the precession of the equinoxes, calculates 

 the solid crust of the earth to have a thickness of 800 or 1,000 miles. 

 Similar investigations by Mr. Hennessey however assign 600 miles as 

 the maximum thickness of the crust. The region of liquid fire being 

 thus removed so far from the earth's surface, Mr. Hopkins suggests 

 the existence of lakes or limited basins of molten matter which serve 

 to feed the volcanos. 



Now the mode of formation of the primitive molten crust of the 

 earth would naturally exclude all combined or intermingled water, 

 while all the sedimentary rocks are necessarily permeated by this 

 liquid, and consequently in a condition to be rendered semi-fluid by 

 the application of heat as supposed in the theory of Scrope and 

 Scheerer, If now we admit that all igneous rocks, ancient plutonic 

 masses, as well as modern lavas, have their origin in the liquefaction 

 of sedimentary strata, we at once explain the diversities in their com- 

 position. We can also understand why the products of volcanos in 

 difi'erent regions are so unlike, and why the lavas of the same volcano 

 vary at different periods. We find an explanation of the water and 

 carbonic acid which are such constant accompaniments of volcanic 

 action, as well as the hydrochloric acid, sulphuretted hydrogen and 

 sulphuric acid, which are so abundantly evolved by certain volcanos. 

 The reaction between silica and carbonates must give rise to carbonic 

 acid, and the decomposition of sea-salt in saliferous strata by silica in 

 the presence of water, will generate hydrochloric acid, while gypsum 



