290 HISTORICAL GEOLOGY 



written records, old monuments and ruins, legends, and such 

 things. Of geologic events there is almost no written record. 

 We must depend upon (1) the testimony of facts afforded by 

 the study of the rocks, and (2) our ability to interpret them 

 according to the natural laws which control everything that 

 happens in the universe. 



Record of physical changes in the earth. When care- 

 fully studied, the rocks may be made to tell much about their 

 own history. Take, for example, the conditions represented in 

 the diagram (Fig. 302). It is evident that the lower rocks 

 have been folded, whereas the upper layers have not. Moun- 

 tains were doubt- 

 less formed by the 

 folding, but the un- 

 conformity shows 



FIG. 302. Ideal Bection of sedimentaiy rocks. that they Were 



worn away, leav- 

 ing a nearly flat surface upon which the sand, which was to 

 become the sandstone, was laid down. The sandstone is evenly 

 stratified, as if the sand had been assorted by currents of 

 water; and if the shells of marine animals are found embedded 

 in the sand, they indicate that the sand was spread out on the 

 bottom of the sea. If the sandstone is now hundreds of feet 

 above the sea and many miles inland, as is true in many cases, 

 it indicates that notable changes have taken place in the dis- 

 tribution of land and sea and in the height of the land above 

 the sea. From a single section of rock it may thus be possible 

 to learn much that has happened in its vicinity in the ages long 

 past. From many such fragmentary records as this, a fairly 

 continuous story of changes of land and sea, mountain growths, 

 base leveling, and other events has been worked out, espe- 

 cially for the latter part of the earth's history. 



Record of changes in living things. On first thought the 

 different kinds of animals and plants about us seem as change- 

 less as the hills and continents. Each bird and each tree, for 

 example, produces young almost exactly like itself, so that the 



