GEOLOGY. 303 



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

 altered fossiliferous strata. My conjectures are now confirmed by the ex- 

 periments of Daubre'e, which serve to complete the demonstration of my 

 theory of the normal metamorphism of sedimentary rocks by the interposi- 

 tion of heated alkaline solutions. 



But to return to the question of intrusive rocks : Calculations based on 

 the increasing temperature of the earth's crust as we descend, lead to the 

 belief that at a depth of twenty-five miles the heat must be sufficient for 

 the igneous fusion of basalt. The recent observations 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 ig- 

 nited, solid from great pressure. This conclusion accords with the math- 

 ematical deductions of Mr. Hopkins, who, from the precession of the 

 equinoxes, calculates the solid crust of the earth to have a thickness of 800 

 or 1000 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 conse- 

 quently 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 composition. "We can also understand why the pro- 

 ducts of volcanoes in different 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 vol- 

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

 sulphuric acid, which are so abundantly evolved by certain volcanoes. 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 in the same way will 

 evolve its sulphur in the form of sulphurous acid mixed with oxygen. The 

 presence of fossil plants In the melting strata would generate carburetted 

 hydrogen gases, whose reducing action would convert the sulphurous acid 

 into sulphuretted hydrogen; or the reducing agency of the carbonaceous 

 matters might give rise to sulphuret of calcium, which would be in its turn 

 decomposed by carbonic acid or otherwise. The intervention of carbona- 

 ceous matters in volcanic phenomena is indicated by the recent investi- 

 gations of Deville, who has found carburetted hydrogen in the gaseous 

 emanations of the region of Etna and the lagoons of Tuscany. The am- 

 monia and the nitrogen of volcanoes are also in many cases probably derived 

 from organic matters in the strata decomposed by subterranean heat. The 

 carburetted hydrogen and bitumen evolved from mud volcanoes, like those 

 of the Crimea and of Bakou, and the carbonized remains of plants in the 

 moya of Quito, and in the volcanic matters of the Island of Ascension, not 

 less than the infusorial remains found by Ehrenberg in the ejected matters 

 of most volcanoes, all go to show that fossiliferous sediments are very gen- 



