152 The American Geologist. September, i9oa 
Geolog'y treats of the world. In order to liave more than a 
superficial knovvled,2^e of geology, it is necessar}^ to know about 
the elements which compose the world, how force acts upon 
tb.ese elements, what aggregates are formed by the elements 
and forces, and how life has modified the construction of the 
world, ("hcmistry teaches of matter ; how it is made up, both 
in life and in death. Without an understanding of its princi- 
ples we cannot have an insight into the constitution of the earth 
or of any part of it. Physics teaches of the manner in which 
the many forms of that strange something we call force acts 
upon matter. Without a knowledge of its principles we can 
never understand the transformation through which the world 
bas gone. The elements which compose the earth are combined 
under th.e laws of physics and chemistry into those almost 
life-like bodies which we call minerals. The minerals are 
commingled in various ways in the rocks. Without a knowl- 
edge of mineralogy no one can have even a superficial under- 
standing of the constitution of rock masses. Biology teaches 
of the substances alive which clothe the outer part of the earth. 
Life is one of the most fundamental of the factors controlling 
the geological transformations in the superficial belt of weather- 
ing ; it has acted as the greatest precipitating agent in the sea. 
Life has had, therefore, a profound and far-reaching effect in 
determining the nature of the sedimentary formations. 
The sciences of chemistry, physics and biology have been 
built up by using minute parts of the materials of the world. 
If geology, or a science of the earth, is to be constructed, it 
must apply to the earth as a whole the principles which have 
so enlightened us as to the nature and relations of the fractions 
of the world which we observe and handle in our laborator- 
ies of physics and chemistry and biology. 
It thus appears that geology is a composite science. It 
might, in a certain sense, be called an applied science. Indeed, 
I have often defined geolog>' as the application of the princi- 
ples of astronomy, of physics, of chemistr}**, of mineralogy and 
of biology to the earth. 
Certainly the earth is the single enormous complex aggre- 
gate of matter directly within the reach of man. This highly 
composite earth is the joint result of the work of astronomical, 
physical, chemical, and biological forces, working on an in- 
