118 LAND MAMMALS IN THE WESTEKN HEMISPHERE 



again filled the great valley of California, except in the northern 

 part of what is now the Sacramento Valley, where there was 

 an accumulation of continental deposits. The immense thick- 

 ness (5000 to 7000 feet) of the California Miocene is largely 

 made up of volcanic material, which testifies to the great 

 activity of the vents. In the Sierras, the height of which was 

 increased in the upper Miocene, there was also a great display 

 of vulcanism, recorded in the lava-flows and tuffs of the time. 

 In the region of Lpwer California and northwestern Mexico 

 considerable changes of the coast-line took place during the 

 Miocene ; in the earlier half of the epoch the Gulf of Cah- 

 fornia was much shorter and narrower than it is to-day and 

 the peninsula was broadly united with the mainland to the 

 east as well as to the north. A wide submergence marked 

 the upper Miocene, reducing the peninsula to a long, narrow 

 island and enlarging the gulf considerably beyond its present 

 limits, flooding an extensive area in northwestern Mexico and 

 sending a small bay into southeastern California. There were 

 great disturbances in the course of the epoch, for in the Santa 

 Cruz Mountains near San Francisco the lower Miocene strata 

 were crumpled into folds, before those of the upper Miocene 

 were deposited upon them. British Columbia, Washington 

 and Oregon were invaded by the sea, which extended up the 

 valley of the Columbia River and its southern tributary, the 

 Willamette, though here the beds are far thinner than those 

 of California. Much of Alaska, both on the north and west 

 coasts and in the valley of the Yukon, was submerged, and the 

 land-connection with Asia appears to have been broken. This 

 is made probable not only by the submergence of the Alaskan 

 coast, but also by the fact that the marine animals of the Cali- 

 fornia coasts and shoal waters, which could not migrate across 

 the ocean, were quite unlike the contemporary forms of the 

 eastern Asiatic shore, which would hardly have been the case, 

 had a continuous coast-line united the two continents. On the 

 other hand, there was a renewed connection with Europe, as 



