584 HISTORICAL GEOLOGY 



action of the weather so long as to be disintegrated to great depths 

 before the overlying lava was outpoured. Lake beds, in one case 

 1000 feet thick, also rest upon one sheet and are covered by another. 

 Although the Snake and Columbia rivers have canyons that reach 

 a depth of several thousand feet, they have not yet succeeded in cut- 

 ting their way to the base, except where they encounter the summits 

 of the mountains buried beneath the flood of molten rock, or near the 

 margin of the flow where it is thinnest. 



Near the edge of the lava plateau water is sometimes obtained 

 from artesian wells, which have been sunk to the sheets of sand and 

 gravel spread by rivers from the surrounding mountains upon the 

 earlier lava flows whose surfaces were afterwards covered by late 

 lavas. However, no water can be obtained in this way over large 

 areas, because the porous lava permits the water to percolate down 

 to great depths, where it appears as springs far down in the canyons. 

 Because of the constant and uniform supply of water thus obtained, 

 the volume of the rivers fluctuates less than in almost any other 

 part of the continent. 



Miocene of Other Continents. — The seas that overspread Ger- 

 many and Belgium in the Oligocene were withdrawn during the Mio- 

 cene, but those of southern Europe not only remained extensive, but 

 were so increased in size as to make that region an archipelago. With 

 the exception of bays in Portugal and France and the submergence of 

 the low lands bordering the North Sea, the shores of western Europe 

 appear to have extended further west than now. Southern Spain was 

 joined to Africa, probably by a wide land connection, but was, in turn, 

 separated from northern Spain by a strait. An important and ex- 

 tensive sea stretched from Vienna to the region of the Black and Aral 

 seas. 



The Miocene was a period of great mountain building in the Old 

 World as well as in the New. The Alps were upheaved and reached 

 nearly their present altitude at this time. The elevation which pro- 

 duced them excluded the sea and formed basins in which rested in- 

 land seas and lakes where are preserved a record of the terrestrial life 

 of the time. The Apennines were reelevated late in the Miocene; 

 and the Caucasus, on which Miocene strata occur at altitudes of 

 6000 feet, also date from this epoch. The Himalayas were raised 

 either at this time or in the Eocene. 



Volcanism, so stupendous in North America at this time, 

 seems to have been of little importance in Europe, although some 



