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. However great its physical advances
may be, Geology remains imperfect till it is wedded with
Palæontology,[2] a study which essentially belongs to the vast
complex of the Biological Sciences, but at the same time has its
strictly geological side. Dealing, as it does, wholly with the