XVI 
INTRODUCTORY OBSERVATIONS. 
may have taken place, without affecting, in any 
sensible degree, the whole mass of the globe. If 
these facts be duly considered, the mind will no 
longer refuse its assent to the interpretation which 
modern geology offers of those natural records of 
its physical history, that are to be found in every 
known region of the earth ; and it will be prepared 
to believe, that the rocks composing the highest 
mountains have been formed in the depths of the 
ocean, and raised to their present situations by 
earthquakes, at various, and, in many instances, 
comparatively very recent, periods. At every step 
the geological enquirer will find, in the displace- 
ments and fractures of the strata, evidence that, 
from the remotest eras, the earthquake and the 
volcano have been in active operation ; and in the 
beds of gravel, and other accumulations of water- 
worn materials, equally decisive proofs of diluvial 
agency. 
The mineral masses of which the crust of the 
globe is composed, may be separated into two prin- 
cipal divisions ; namely, the jyrimary and the se- 
condary, The primary are destitute of organic 
remains, and occupy the lowermost place in the 
order of superposition of the strata; yet, having 
manifestly been injected from below, they also 
very generally form the summits of the highest 
peaks and mountain chains in the world. Those 
which are decidedly of igneous origin, are granifey 
sienite, porphyry ^ basalt^ &c. They were called 
primary, because it was supposed, from the ab- 
sence of fossils, that they were formed before the 
creation of animals and vegetables ; but it is now 
