INTRODUCTION 3 
This condition has led many who have given cursory 
glances at geology, to suppose that it is a science of 
guesses: it is rather to be looked upon as a fortunate 
illustration of the fact, that there are some things in 
creation which the minds of even the best thinkers 
cannot explain. We are entirely too apt to believe 
that everything worth knowing about the universe is 
already understood, and to confound theory with fact. 
Here in this subject we shall find many things that we 
do not know; and perhaps some of them will always 
remain unknown. 
The earth’s study is a difficult one for the person who 
would read from nature itself. In such a study we 
are usually able to see only the merest film of rock at 
the surface. Here and there the upturned strata of a 
mountain, or the layers through which a river has cut. 
its gorge, or a mine has pierced, will give us a glimpse 
of the conditions below. But a mile is the deepest 
that we have penetrated, and nearly everywhere our 
studies are confined to the very surface, if, indeed, even 
this is not forbidden us by a thick soil covering. We 
can study to a depth of only a mile: then how can 
we tell what is the condition of the other 7900 miles? 
Naturally, if we would form an idea concerning these 
conditions, we must speculate, and some of our theories 
are confessedly little more than guesses. 
While there are certain comparisons which may 
