54. ELEMENTARY GEOLOGY 
The rocks of the earth’s crust are formed in one of 
five ways: (1) by the solidification of molten rock, as 
the lavas; (2) by chemical precipitation from water, as 
illustrated by the beds of salt; (3) by the action of 
animals or plants, as in the case of coral and coal 
strata; (4) by the mechanical destruction of other rocks, 
as in the sand and clay beds; 
and (5) by the alteration, or 
metamorphism, of one of these 
classes of rock, as in the case 
of marble. ‘The first are called 
igneous rocks, the second, third, 
and fourth are grouped as sedi- 
mentary, and the fifth as meta- 
morphic. It is only the first 
of these that are considered in 
this chapter. 
Origin of Igneous Rocks. — 
Ginschiancasteleoh tasae Gem As is stated in Chapter XVIII, 
pit, from which loose gravelis there are places in the earth 
being taken. 
Fia. 13. 
from which molten rock rises 
to the surface, from some point within the crust. 
Usually the lava rises through a tubular vent and 
builds a volcanic cone, though sometimes it wells 
up through fissures and flows away as great floods. 
Deep down in the earth these lavas are rising, but, 
failing to reach the surface, are spreading out between 
