44 THE AGE OF MAMMALS 



may in some instances prove impossible, owing to the absolute independ- 

 ence of the movements of the earth and of the other physical phenomena 

 which caused these stages in the Old and New Worlds. It is obvious that 

 the overlapping in time of these minor periods of deposition would be the 

 rule and that exact synchronism would be largely coincidence and there- 

 fore highly improbable; all that we can reasonably hope to establish in 

 the near future is approximate synchronism of the stages. Ultimately the 

 lines of time overlap may be determined. 



Time Value of Fossils 



During the Age of Mammals we should endeavor to establish absolute 

 time in different parts of the world, like Greenwich standard time of to-day, 

 not through measuring the thickness of the rocks but through using as 

 our chronometers all the known forms that lived, plants, and vertebrate 

 and invertebrate animals. The thickness of the rocks varies enormously, 

 and is correspondingly deceptive. The fresh-water Oligocene rocks of the 

 western plains, for example, are only 400 to 800 feet in thickness, while 

 on the Pacific coast and in Italy marine rocks of the same age are 10 to 

 12,000 feet in thickness. The thickness of rocks is one of the means of 

 estimating the total duration of the Age of Mammals, while the stages of 

 evolution in animals and plants give us the punctuation points, as it were, 

 or the means of keeping geologic time. It is true that during the Cseno- 

 zoic Era the plants are comparatively stationary, and so are the amphibians,, 

 fishes, and reptiles, but the mammals are in a state of continuous and in- 

 cessant change, and what gives them especial chronometric value is that 

 the rate of change or of evolution is the same in many parts of the world 

 at the same time. Even during the Age of Reptiles we may take advan- 

 tage of the remarkably constant evolution of the herbivorous multituber- 

 culate gnawing mammals known as Plagiaulacidse, surviving members of 

 which are found in the Basal Eocene (Fig. 28). The grooves on the sides 

 of the large cutting teeth of Plagiaulax and the cusps, or tubercles, on the 

 grinding teeth are successively added with the precision of clock-work, 

 while the number of premolars is diminishing. If we suppose the rate 

 of evolution has been about the same, we can approximately calculate the 

 intervals of deposition.^ 



Age of Reptiles Age of Mammals 



Stonesfield Purbeck Laramie Puerco Cernaysian 



Diminishing number of pre- 

 molars ? 4-3 2 2-1 1 



Increasing grooves on pre- 

 molars ? 7-9 11-14 12-15 14 



Increasing number of molar 



tubercles: outer; inner . ? 4:2 6:4 6:4 9:6 



' See Osborn, H. F., The Rise of the Mammalia in North America. Proc. Amer. Ass. Ad. 

 Sci., 1894. pp. 188-227; and Amer. .lour. Sci., Nov. and Dec, 1893. 



