CONSTRUCTION OF THE EARTH. 125 



as you have already seen in 180, and will see more fully 

 hereafter. 



The geologist finds a rock which contains imbedded in 

 it certain shells. These shells were, like the shells we 

 now pick up on the sea-shore, once inhabited by animals. 

 But how did they get into the rock? This question is 

 easily solved by the geologist. He finds shells at the 

 present -time imbedded in mud or sand, the mineral char- 

 acter of which is precisely the same with that of the solid 

 rock containing the other shells. He then justly infers 

 that the latter were once in similar mud or sand, which 

 is now solidified. 



He makes a farther inference in regard to this rock by 

 comparison with some other. Some of its shells may be 

 of the same species with some that are found at the pres- 

 ent day, while those found in the rock with which it is 

 compared may all be different from any of the present 

 species. His inference is that the latter rock belongs to 

 an earlier age than the former. It may have been made 

 or solidified into rock hundreds, or even thousands of 

 centuries before the other, though the two may now lie 

 in juxtaposition. The kind of observation here indicated, 

 you will find as we proceed, is largely made use of in de- 

 termining the relative ages of rocks ; for as, during al- 

 most all the ages of the earth, there have been living be- 

 ings, but differing in character from age to age, the re- 

 mains of life found in the rocks differ according to the 

 ages in which the rocks were formed. 



Take another case. You often see in a block of sand- 

 stone a step perhaps at some door pebbles imbedded 

 in the material of which most of the rock was made. 

 These pebbles are such as you have seen on a shore, and 

 you know that they once were rough pieces of rock, and 

 that they were smoothed by being rubbed together a long 

 time, as those you see on the shore have been, by water 

 rushing over them, and that after this was done they be- 

 came mingled with sand, and the whole became a solid 



