672 THE POPULAR SCIENCE MONTHLY. 



the Eocene period, the first of the three divisions of the Tertiary 

 era. They were contemporaneous with the growth of the Uinta 

 range and the Junction and Yampa Mountains ; "but the Henry 

 structure represents sudden lifting by the energy of volcanic 

 inflows of molten rock, while the Uinta structure, as we have 

 seen, represents a very gradual upheaval. The two can not be 

 referred to the same means of elevation, though their more remote 

 causes were doubtless nearly related or identical. No laccolite 

 mountains are known in other countries, and here they are found 

 only in the region of plateaus which is intersected by the canon 

 of the Colorado. 



4. Tilted Mountain Ranges. Next to the west of the Colo- 

 rado drainage area is the Great Basin of interior drainage, which 

 returns all its rainfall again to the clouds by evaporation. Were 

 the lakes of this arid region to grow by increased rainfall until 

 they should flow across the lowest points of their water-sheds and 

 send streams to the ocean, two of them would be similar in area 

 to the Great Lakes of the St. Lawrence. Twice during the climatic 

 changes of the Glacial and Post-glacial epochs, these two lakes, 

 named Bonneville and Lahontan, have so risen nearly or quite to 

 overflowing, whereas now the former is represented by Great Salt 

 Lake in Utah, and the latter by Pyramid and Winnemucca Lakes, 

 with others in Nevada. Close east of Lake Bonneville rises the 

 "VVahsatch range, and west and southwest of Lake Lahontan is 

 the Sierra Nevada, both of which are examples of tilted mountain 

 ranges. The Wahsatch has been elevated along fault lines which 

 form its western boundary, adjacent to the area of Lake Bonne- 

 ville and the present Great Salt Lake. It is an immense mount- 

 ain mass which has been tilted by upheaval of its western border 

 and sinking of its eastern portion. The Sierra Nevada, on the 

 other hand, has been upheaved along fault lines bounding it on 

 the east, and is concisely described as principally a single great 

 block of the earth's crust, about three hundred miles long from 

 north-northwest to south-southeast, and fifty to seventy miles 

 wide, tilted by elevation of its east side and depression of its west 

 side. Between these grand mountain ranges which look toward 

 each other on the east and west limits of the Great Basin, many 

 minor ranges occur, trending from north to south, all of which 

 have the same structure and origin through faulting and tilting, 

 so that this is called by Powell the Basin type of mountain 

 structure. 



The great disturbances producing the Basin ranges were of 

 late geologic date, in the early part of the Quaternary era. The 

 resulting mountain ranges are still very young, geologically 

 speaking, and therefore some of them rank among the most promi- 

 nent on this continent. During more remote periods doubtless 



