Page:Popular Science Monthly Volume 39.djvu/690

672 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.

—Next to the west of the Colorado 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 WahsatchWasatch [sic] 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 Bonneville and the present Great Salt Lake. It is an immense mountain 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 prominent on this continent. During more remote periods doubtless