FACTS ABOUT MOUNTAIN REGIONS. 785 



did not take the larger part of their elevation until the Tertiary era 

 had opened. The question whether the first existence of continents 

 and oceanic depressions may not be explained by a reference to the 

 earth's contraction is not here considered. 



2. FACTS ABOUT MOUNTAINS. 

 (1.) Characteristics of Disturbed Regions and Mountains. 



Changes of level have sometimes taken place on a grand scdle with- 

 out appreciable change in the positions of the rocks of the region 

 moved. Such were the oscillations in the Quaternary over the higher 

 latitudes, and other movements which have given elevation or depres- 

 sion to large portions of continents at once. But over all the con- 

 tinents, the rocks almost everywhere bear evidence of the action of 

 disturbing forces, both in their inclined positions, and in fractures ; and 

 over large portions the beds stand at high angles. 



These disturbed conditions of rocks include (1) flexures of all 

 grades, from the feeblest bulgings of great areas to the bending up of 

 strata into many steep successive folds ; fractures, from local cracks to 

 those that descend to indefinite depths ; faults, or displacements along 

 fractures, of all amounts, up to 20,000 feet or more. Mountain regions 

 are regions of disturbance ; in them occur all degrees of folding, frac- 

 turing, and faulting, from the feeblest to the most extreme. An ex- 

 planation of the origin of mountains is to a large extent, therefore, a 

 discussion of the nature and origin of these phenomena. 



1. Flexures. — The characteristics of the flexures of rocks are il- 

 lustrated in the preceding pages on Historical Geology. On 93-96, 

 98, and 101, are representations of some of the forms ; these occur 

 with a breadth of a few feet, or a span of many miles. Others are 

 shown on pages 213-215, from examples in the Green Mountains, and 

 on page 396 from the Appalachian range. The following section of a 

 part of the Appalachian region of Virginia, by Prof. J. L. Campbell, 

 is here introduced as a further illustration of the subject. It represents 

 the rocks for a breadth of thirty miles across the range, from a point 

 southeast of Rockbridge Baths to two miles northwest of Warm 

 Springs in Bath County, Virginia. The vertical heights are marked 

 on the side. The numbering of the formations corresponds with that 

 on page 142; the limestone areas in the cut are blocked, the shales 

 ruled, and the sandstones dotted. (For particulars as to the geology 

 of the region, see the memoir by Prof. Campbell, Amer. J. Sci., III., 

 xviii., 119, 1879.) Farther to the southeast, in the same line, the folds 

 are more closely crowded, and become a series of nearly vertical 

 beds. The Reports of the first and second Geological Surveys of Penn- 

 sylvania contain many figures of Appalachian flexures. 

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