FOSSIL rOKESTS OF YELLOWSTONE PARK. 



27 



The deciduous-leaved trees are almost a negligible element in the 

 present park flora, being confined to an occasional cottonwood 

 {Popidus angustlfolia) at the lower elevations, along the Yellow- 

 stone Eiver, and small groves of the quaking aspen {Populus 

 tremuloides) . Along the streams and in wet places there are many 

 species of willow (Salix) and several alders (Alnus), and in moun- 

 tain bogs and valleys there is a small birch {Betula glandulosa) . 

 There are, of course, many small shrubs, such as gooseberries, 

 currants, and roses. 



AGE OF THE FOSSIL FORESTS. 



The question is often asked. How old are the fossil forests? It is, 

 of course, impossible to fix their age exactly in years, though it is 

 easy enough to place them in the geologic time scale. The stratified 

 rocks that make up the crust of the earth, from the oldest we know 

 to the most recent, have been divided by geologists into a number 

 of major divisions or systems, each — except perhaps the oldest- 

 containing the remains of certain kinds of plants and animals. 

 The accompanying diagram (fig. IG), shows these major time divi- 

 sions, arranged in their proper sequence from the lowest to the 

 highest. The star in this geologic time scale indicates the age 

 of the rocks in which the fossil forests were entombed. It shows 

 that they were buried during the Tertiary period. This period is 

 divided into four epochs, the oldest called Eocene, having been suc- 

 ceeded in turn by the Oligocene, the Miocene, and the Pliocene, which 

 just precedes the Pleistocene or glacial epoch. The forests of the 

 Yellowstone National Park are found in the Miocene series of the 

 Tertiary. As compared with the eons of geologic time that preceded 

 it the Miocene is relatively very recent, though, if the various esti- 

 mates of the age of the earth that have been made by geologists are 

 anywhere near correct it may well have been a million years ago. 

 It must be remembered, however, that this estimate involves more or 

 less speculation based on a number of factors which may or may not 

 have been correctly interpreted. 



A study of the fossil trees themselves gives at least a rough ap- 

 proximation as to the length of time it may have taken to accumu- 

 late the beds in which they are now buried. As already mentioned, 

 there is a succession of forests, one above another, through a thick- 

 ness of 2,000' feet of strata. The unit of the measure of the time is 

 the time taken by each forest to grow. Pine trees of the types repre- 

 sented in the fossil trunks require 200 or 300 years to reach maturity, 

 and redwoods may require from 500 to 1,000 years. Twelve or more 

 of these forest levels have been found. By multiplying this number 

 by the minimum age of the trees (200 years) we shall have 2,400 

 years, and by multiplying it by the maximum age of the redwood 



