4 ELEMENTARY GEOLOGY 
fairly be made between geology and human history, 
there are also striking differences. There was a 
time —and indeed it continued down to the memory 
of some geologists still living — when it was believed 
that the history of the earth had all taken place within 
a few thousand years. Then great difficulties were 
encountered in the attempt to reconcile with this pre- 
conception, the facts so plainly stamped on the face of 
the rocks. The only way in which this was possible, 
lay in supposing that the real earth history had been 
one of great and frequent catastrophes, and that the 
ordinary conditions of the present were introduced only 
after that history had practically ended, and the earth 
had reached its present form. 
Until geologists were driven from this belief by the 
accumulation of unanswerable facts, there was no real 
science of geology. At the beginning of the book it 
is impossible to state the proofs of the immense age 
of the earth; but it is hoped that as the pages are 
turned, this truth may become more and more firmly 
rooted in the reader’s mind. Although as yet no one 
is ready to say how old the earth is, geologists are 
united in the belief that its age is to be represented 
in millions of years; and no one can study geology 
intelligently who is not ready to grasp this great and 
fundamental principle. 
Geology is so young that some educated people are 
