58 THE AGE OF MAMMALS 



origin and may contain the intermingled remains of land and sea animals. 1 1 . Phos- 

 phorites (phosphate of lime) {"Phosphorites," " Phosphorit"). Also of organic 

 origin, direct!}' or indirectly derived from the hard parts of animals, or from the 

 excrements of animals. 12. Asphalt or asphaltum {" Asphalte," "Asphalt.") The 

 residumn of pitch lakes left by the evaporation of petroleum springs. A remark- 

 able asphaltum deposit (Rancho La Brea, see Fig. 205) has recently been found in 

 the Pleistocene of southern California, containing a rich variety of mammals in 

 remarkable preservation. 13. Breccias {"Brechc," "Breccia"). Formed by the 

 filling in of bones and gravels cemented together by calcareous waters. 



V. Duration of the Age of Mammals 



How long was the Age of Mammals? How many years ago did it 

 begin? How may we find out? If we remark, for example, that Eohippus, 

 the first stage in the development of the horse, was an animal which lived 

 about three millions of years ago, our hearer looks incredulous and has a 

 perfect right to ask, What are your grounds for assigning such an enormously 

 long period of time? There are a great many ways of estimating geologic 

 time, all of which either depend on the comparison of past processes with 

 present processes of earth formation, or make an appeal to astronomic 

 data, such as the procession of the eclipses, the eccentricity of the earth's 

 orbit, or the consolidation of the earth's crust and the period necessary for 

 cooling sufficiently to admit of life. 



A vast period. — Whatever method of calculation we adopt, a glance 

 at the accompanying diagram shows that the Age of Mammals, while 

 vastly long in itself, was relatively short as compared with all the life periods 

 which preceded it; it was estimated by Dana in 1874 as occupying only 

 one-sixteenth of the whole life period, by Wallace in 1895 as occupying one- 

 twentieth of the whole life period. Each of these Ages represents a vast 

 interval, as attested both by the great geographic changes which occurred in 

 them, by the great mountain chains which were thro\A^n up and then com- 

 pletely reduced to the general level, by the enormous thickness of the sedi- 

 mentary deposits which were laid down on land and sea, recently estimated 

 at a total of 265,000 feet or upwards of 50 miles (Sollas, 1900), or 335,800 

 feet (Sollas, 1909), and still more perhaps by the great changes in the ani- 

 mal and plant life which are recorded in the fossils/ 



Mountain births. — Biologists from Darwin to the present time have 

 demanded long periods for these evolutionary changes and for the Age of 

 Mammals itself. As a measure of the lapse of time the comparison of the 

 great advance in size and structure between the Eocene Eohippus and the 

 existing horse (Equus) (Fig. 14) is perhaps less impressive than a revicAV 

 of the great mountain births which occurred during the Age of Mammals. 



The Rocky Mountains, it is true, began their elevation during the close 



' See Poulton, 1896, A Contribution to the Discussion of the Age of the Earth, Essays on 

 Evolution, 1908, p. 15. 



