CENOZOIC ERA: AGE OF MAMMALS 



583 



of forms that belong to a previous epoch in other parts of the earth, just as in an 

 age of electricity and cement some isolated Indian tribes are still living in the Stone 

 Age. 



Igneous Activity. — Perhaps no other period in the history of the 

 earth since Pre-Cambrian times displayed such extraordinary volcan- 

 ism as the Tertiary, and of the four epochs of the period, the Miocene 

 was by far the most important in this particular. 



It has already been seen that the great volcanic outbursts of the 

 Pacific coast occurred during the Miocene — especially during the 

 middle of that epoch — covering 

 that region of North America 

 with ash which furnished the 

 material for a great thickness of 

 sedimentary deposits. Not only 

 on the Pacific coast, but perhaps 

 in every state west of the Rocky 

 Mountains, some evidence of the 

 igneous activity of this time can 

 be found. It was during this 

 period that a great quantity of 

 lava and ash was poured into 

 the basin of the Yellowstone 

 National Park. Some of the 

 forests that were buried in the 

 ash at that time were later petri- 

 fied and have been partially un- 

 covered by erosion. Seventeen 

 such petrified forests, one above Amethyst Mountain, Yellowstone National 

 the Other, may be seen in one P ark - Seventeen or more successive forests 

 (V r-2T^ were covere d with volcanic ash and the 



sec O ^ g. 53 ;. ^ logs petrified. About two thousand feet of 



The greatest area of lava in stra ta are shown. (After W. H. Holmes.) 

 North America covers a region of 



between 200,000 and 300,000 square miles in Washington, Oregon, 

 Idaho, and California (Fig. 304, p. 311), and is known, from the ex- 

 posures on faulted and tilted blocks, to have a maximum thickness of 

 at least 5000 feet. By far the largest bulk of this was outpoured 

 during the Miocene. This enormous mass of lava was built up by 

 successive lava flows averaging about 75 feet in thickness. On the 

 canyon walls some of the sheets are seen to be separated by old soil 

 beds, showing that the former lava surface had been exposed to the 



