CHAPTER LXXVII 

 DISTRIBUTION IN TIME THE GEOLOGICAL RECORD 



If a large and complicated tree were submerged in water 

 except the ends of some of its branches, these ends, projecting 

 above the surface, would have the same sort of relationship to 

 one another as existing groups of animals. To sketch the sub- 

 merged tree on the evidence of the parts seen above water would 

 prove a hopeless task, and to determine the mutual affinities of 

 existing groups of animals without knowledge of their past history 

 is also a difficult matter, though structure and development give 

 many clues. Geology, however, furnishes us with a great deal of 

 material from which to reconstruct the ancient life-history of the 

 earth. It deals with periods of which the oldest date back to 

 immensely remote times, if measured by the ordinary human 

 standards of years and centuries, and the history of mankind 

 occupies only the last page of the chronicle. 



THE GEOLOGICAL RECORD. At the present time deposits of 

 sand, mud, and limestone are being formed in the sea, in bodies 

 of fresh water, and some other places, and these enclose the 

 remains of dead animals, such as are well provided with hard 

 parts standing the best chance of preservation. These deposits 

 are arranged in successive layers, of which the uppermost are 

 necessarily the youngest, and contain the remains of such animals 

 as have most recently deceased. Examination of the hard frame- 

 work of the land shows that a large part of it is made up of rocks, 

 such as clay, slate, sandstone, and limestone, which are similarly 

 arranged in layers, i.e. are stratified^ each such layer being known 

 as a stratum (fig. 1312). Imbedded in these strata are fossils, 

 which are no other than the remains of animals (and plants) 

 which once existed, or markings, e.g. footprints and other im- 

 pressions, that prove the existence of certain forms of life at the 

 time when the rocks containing them were formed. These strati- 



456 



