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, w^hich 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 
