510 



THE THEORY OF EVOLUTION 



appearing in the fossil-bearing rocks. In view of the manner in 

 which sediments are deposited in water, it is a truism to say 

 that the oldest layers or strata are at the bottom and the youngest 

 at the top, when such deposits have been elevated above sea level 

 and hardened into rock with no disturbance of their horizontal 

 relationships. In many places this elevation has occurred without 



Fig. 272. — Mammoth found frozen in a cliff in Siberia. 



Above, the specimen partially uncovered showing skvill and fore feet; below, the specimen 

 as mounted in the Zoological Museum at Leningrad. The animal had slipped into a 

 crevice from which it was unable to extricate itself and been covered with snow, which 

 hardened into ice, before the flesh could decompose. (Adapted from Herz, Report Smith- 

 sonian Institution, 1903.) 



such disturbance (c/. Figs. 273 and 274) . In such cases it may be 

 possible to recognize not only the fossils but traces of the ancient 

 topography, as when there are signs of beaches, mud flats, or deep 

 water. In other parts of the earth, particularly in great mountain 

 ranges, strata that were originally horizontal have been folded 

 or broken as faults during their elevation. Although the con- 

 ditions presented are often perplexing after folds some thou- 

 sands of feet in thickness have been worn away by erosion, the 



