124 GEOLOGY. 



mentary ruins of rocks being the materials out of which 

 a large portion of the rocks we now find were construct- 

 ed. You will see that, although at the first there were 

 ages in which there was no life, vegetable or animal, there 

 was a long series of ages before man appeared in which 

 the earth swarmed with life, the relics being found now 

 imbedded in the rocks, differing from each other, howev- 

 er, in a marked manner, as geologists have found, in the 

 different stages of the earth's construction. It is from 

 these differences in the forms of life that geologists have 

 been able to mark out the periods or ages of the world's 

 formation. In the present chapter it is my intention to 

 point out some of the processes by which the earth has 

 been gradually built up into its present condition, as pre- 

 paratory to the consideration of what took place in each 

 of its several ages. What you have already learned of 

 the present changes going on in the earth, you will find, 

 will throw much light upon this subject, because, as I 

 have before stated, the same agents which are at work 

 now in these changes did the work in the changes of the 

 far past. 



210. Nature of Geological Evidence. The geologist 

 takes the results of processes which are now going on in 

 the earth, and, comparing them with the results which he 

 finds buried up in the rocks, adopts his conclusions in re- 

 gard to the latter, and does so without danger of error 

 if he be properly cautious. I will give a few illustrations 

 of his modes of reasoning. 



The geologist finds a rock which, on examination, is 

 discovered to be of the same composition with clay, and, 

 more than this, has similar layers. He infers that the 

 rock was once clay, and in some way became solidified 

 or changed into rock. By the same reasoning he infers 

 that certain rocks were once mud, and certain others 

 were once sand. The inference in these cases is con- 

 firmed by the fact that the solidification has sometimes 

 been known to take place within a short period of time, 



