8 ELEMENTARY GEOLOGY 
So the history of the earth has been one of slow 
changes like those now in operation; but these have 
been working for such a great length of time that a 
wonderful series of events has taken place. Since this 
is so, it follows that the best way to unravel the past, 
is to appreciate the conditions of the present, and then 
apply the knowledge thus obtained to a study of the 
early ages. We will therefore first look at the earth as 
it is, then as 1t has been. 
It is customary to divide geology into parts, not 
because there is any real separation, but rather for the 
convenience of grouping allied phenomena. Geologists 
differ in what they include under the several divisions, 
showing that there is no real distinction. In this 
book the subject will be divided into three parts: 
(1) Structural Geology, which deals with the materials 
of which the earth is made, without reference to 
their origin. (2) Dynamic Geology, which considers 
the forces that are at work on the earth to modify its 
surface, and the changes which these are producing. 
(3) Stratigraphic Geology, which treats of the earth’s 
past and its development since the beginning of the 
readable history. A fourth division commonly made 
is Physiographic Geology, which treats of the out- 
lines and features of the land, and the history which 
has produced them. A certain amount of physiogra- 
phic geology is included under the divisions of dynamic 
