VOLCANIC ROCKS. 
29 
sils^ so abundantly included in the earth’s crust. By a fos¬ 
sil is meant any body, or the traces of the existence of any 
body, whether animal or vegetable, which has been buried 
in the earth by natural causes. Now the remains of ani¬ 
mals, especially of aquatic species, are found almost every¬ 
where imbedded in stratified rocks, and sometimes, in the 
case of limestone, they are in such abundance as to consti¬ 
tute the entire mass of the rock itself Shells and corals are 
the most frequent, and with them are often associated »the 
bones and teeth of fishes, fragments of wood, impressions of 
leaves, and other organic substances. Fossil shells, of forms 
such as now aboujid in the sea, are met with far inland, both 
near the surface, and at great depths below it. They occur 
at all heights above the level of the ocean, having been ob¬ 
served at elevations of more than 8000 feet in the Pyrenees, 
10,000 in the Alps, 13,000 in the Andes, and above 18,000 
feet in the Himalaya.* 
These shells belong mostly to marine testacea, but in some 
places exclusively to forms characteristic of lakes and rivers. 
Hence it is concluded that some ancient strata were deposit¬ 
ed at the bottom of the sea, and others in lakes and estuaries. 
We have now pointed out one great class of rocks, which, 
however they may vary in mineral composition, color, grain, 
or other characters, external and internal, may nevertheless 
be grouped together as having a common origin. They 
have all been formed under water, in the same manner as 
modern accumulations of sand, mud, shingle, banks of shells, 
reefs of coral,and the like,and are all characterized by strati¬ 
fication or fossils, or by both. 
Volcanic Rocks.—The division of rocks which we may next 
consider are the volcanic, or those which have been produced 
at or near the surface whether in ancient or modern times, 
not by water, but by the action of fire or subterranean heat. 
These rocks are for the most part unstratified, and are de¬ 
void of fossils. They are more partially distributed than 
aqueous formations, at least in respect to horizontal extern 
sion. Among those parts of Europe where they exhibit 
characters not to be mistaken, I may mention not only Sici¬ 
ly and the country round Naples, but Auvergne, Velay, and 
Vivarais,now the departments of Puy de Dome, Haute Loire, 
and Ardeche, towards the centre and south of France, in 
which are several hundred conical hills having: the forms of 
modern volcanoes, with craters more or less perfect on many 
of their summits. These cones are composed moreover of 
* Col. R. J. Stracbey found oolitic fossils 18,400 feet high in the Hima- 
laja. 
