THE 
PoC Te NY RIP See PO RY 
OF 
AES eo BA REE 
INTRODUCTION: 
THe Laws OF GEOLOGICAL ACTION. 
Unpber the general title of “‘ Geology ” are usually included at 
least two distinct branches of inquiry, allied to one another in 
the closest manner, and yet so distinct as to be largely capable 
of separate study. Geo/ogy,* in its strict sense, is the science 
which is concerned with the investigation of the materials which 
compose the earth, the methods in which those materials have 
been arranged, and the causes and modes of origin of these 
arrangements. In this limited aspect, Geology is nothing more 
than the Physical Geography of the past, just as Physical Geo- 
graphy is the Geology of to-day ; and though it has to call in 
the aid of Physics, Astronomy, Mineralogy, Chemistry, and 
other allies more remote, it is in itself a perfectly distinct and 
individual study. One has, however, only to cross the thresh- 
old of Geology to discover that the field and scope of the 
science cannot be thus rigidly limited to purely physical pro- 
blems. The study of the physical development of the earth 
throughout past ages brings us at once in contact with the 
forms of animal and vegetable life which peopled its surface in 
bygone epochs, and it is found impossible adequately to com- 
* Gr. gé, the earth ; Jogos, a discourse. 
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