CENOZOIC ERA: AGE OF MAMMALS 633 



seeds carried there from California. Their extinction becomes more 

 remarkable since it is known that " under the most favorable condi- 

 tions these giants probably live 5000 years or more, though few of 

 the larger trees are more than half as old." One careful observer 

 says, " I never saw a big tree that had died a natural death : barring 

 accidents they seem to be immortal, being exempt from all the diseases 

 that afflict and kill other trees. Unless destroyed by man, they live 

 on indefinitely until burned, smashed by lightning, or cast down by 

 storms, or by the giving way of the ground on which they stand." 

 (John Muir.) 



Diatoms. — The earliest specimens known with certainty to be 

 diatoms are from the Jurassic. This group did not become common 

 until the Cretaceous, or abundant until the Tertiary. A stratum 

 50 feet thick in Bohemia is almost entirely composed of diatoms ; 

 about Richmond, Virginia, there is a deposit 30 feet thick and many 

 miles in extent ; and deposits of diatomaceous earth are common in 

 other parts of the Coastal Plain. Thick beds also occur in the Cali- 

 fornia Tertiary (p. 581). Diatom deposits are now forming in the 

 Yellowstone National Park, where "they cover many square miles in 

 the vicinity of active and extinct hot spring vents of the park, and 

 are often 3 feet, 4 feet, and sometimes 5 to 6 feet thick." In the Ter- 

 tiary, diatom deposits were formed in sluggish streams, in lakes, on 

 the sea bottom, and in hot springs, as they are now in Nevada and 

 California. The Cretaceous and Tertiary species very closely re- 

 semble living forms. 



Exceptional Preservation of Plants. — The preservation of such 

 delicate parts of plants as flowers, catkins of the oak, pollen grains, 

 as well as fungi, in the amber of Oligocene trees, gives us almost 

 as definite knowledge of some of the Oligocene plants as if they were 

 living to-day. 



In certain parts of Europe Oligocene mineral springs covered with 

 their deposits whatever organic remains they touched and thus 

 buried them. After a time the organic matter decayed, sometimes 

 leaving perfect molds of their forms. When these molds are properly 

 filled, casts even of fossil flowers and insects are sometimes obtained. 



Plant Localities in North America. — About 500 species of Miocene 

 plants are known in North America, but the deposits in which they 

 are found are small and widely separated. One at Brandon, Ver- 

 mont, consists of a small, pocket-like deposit of lignite, at one time 

 worked for fuel, which has yielded a large number of fossil fruits, 



