102 



STRUCTUKiU. GEOLOGY, 



to have come from the left, they would be nnderthrnst flexures, — a kind 

 that is exemplified iu some sections of the Alps, but is not common like the 

 overthrust. 



Flexures are either anticlines or synclines. Upward and downward bends 

 alternate, as the figures show ; the upward, lettered A, are anticlines, — 

 so-named from the Greek aVrt, opposite, and kXiVw, incline; and the down- 

 ward, are synclines — from crvv, together, and kAiVw. When strata have been 

 pushed up so as to dip only in one direction, the structure is called mono- 

 clinal, from ynovos, one, and KXivw. One example of a monocline is shown in 

 Fig. 91 (2) . The beds in Fig. 96, on page 104, have a monoclinal position, 

 but they may be either those of a monocline or of anticlines and synclines, 

 as explained beyond. 



As the following figures of actual sections indicate, flexures are not 

 found in nature with their original forms, owing to the wear such regions 

 have always undergone. Fig. 92, by Rogers, represents a section six miles 



YIV Vf 



92. 



mn in VI V T/ivvrriv^iiiif 



Appalachian section, Virginia. Uogers, '42. 



long, from the Appalachians in Virginia. The strata are numbered, so that 

 the flexures of a given stratum may be followed ; thus III bends over II, 

 to the left of the middle of the figure, and the right portion descends to 

 come vip again in III at the right end of the figure ; again, IV, to the left, 

 rises and bends over III and II, though disjoined about the top of the fold 

 by denudation. 



Fig. 93 represents a section from the Swiss side of the central Alps. 

 To the right, the strata, 1 to 6, are so flexed over that the newest stratum 6 is 

 beneath 4, 3, 2, 1, with 1, the oldest, at top. The dotted lines help in 

 tracing out the flexures. Other sections from the Appalachians, the 

 Alps, and other regions, are given under the subject of Mountain-making 

 (pages 355-360). 



93. 



Section east of Lucerne, extending south, 15 m., through Windgiille (4, to the right), a peak 10,455 feet 

 high; 1, Gneiss; 2, Triassic beds; 3, Lias; 4, Jurassic, above the Lias; 5, Cretaceous; 6, Eocene 

 Tertiary, including Nummulitic beds. Heim. 



Besides the apparent irregularities introduced into a region of flexures 

 by denudation, there are others still greater arising from fractures and 

 faults (displacements). Overthrust flexures very commonly become broken 



