INTRODUCTION.
THE LAWS OF GEOLOGICAL ACTION.
Under 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.

_Geology_,[1] 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 Geography 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 threshold of Geology to discover that the field and scope of the science cannot be thus rigidly limited to purely physical problems.

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 comprehend the former, unless we possess some knowledge of the latter.