122 Proceedings of the Royal Physical Society. 



West Indies down the coast of South America and across 

 the Pacific. It has a long line of volcanoes marking its 

 course, and those districts situated over it are the very 

 centres of earthquake violence. 



2. Secondary Formations. — The probable conditions under 

 which the primitive rocks were formed have been already 

 mentioned. The secondary class have evidently originated 

 from the disintegration of the older formations. Two facts 

 are ascertained in regard to the secondary formations — first, 

 that they were arranged in their present state by the action 

 of water ; and next, that they were deposited after the crea- 

 tion of organised beings. And from these facts follows 

 the conclusion that this deposition must have taken place 

 after the temperature of the surface had fallen far below 

 212° Fahr. 



3. Igneous Bocks. — The igneous rocks have always been 

 held as a strong argument in favour of a central heat. They 

 are supposed to consist either of a central fluid matter, 

 or of primitive rocks melted and protruded from below. 

 Their appearance is against the supposition ; but this 

 has been ascribed to the rate of cooling. Basalt or green- 

 stone, if melted and cooled rapidly, forms a perfect glass ; 

 but if cooled slowly, each passes into a gray crystalline 

 stone. 



4. Volcanoes. — By the theory, volcanoes are assumed to 

 be natural vents through which elastic vapours of various 

 kinds make their escape, which might otherwise cause serious 

 derangements in the crust of the earth. Some are supposed 

 to exist over a fissure in the crust, which descends to the 

 molten matter beneath ; others, to have their origin nearer 

 the surface — such are those that produce periodical dis- 

 charges of lava, watery vapour, and the emission of flame. 



5. Earthquakes and Hot Springs. — Various theories have 

 been proposed to account for the phenomena of earthquakes, 

 but the theory of a central heat would appear to afford a 

 more easy explanation than any derived from chemical 

 causes. It is known with what facility water descends 

 between the rocky strata, and if it arrive, as it may easily 

 do, to the point where even water boils — viz., about 10,000 



