740 



DYNAMICAL GEOLOGY. 



geosynclinals, 1 — besides flexures, fractures, and displacements of the 

 overlying strata, and great fractures, shovings, and crushings, where 

 bending has reached its limit. 



Fig. 1122. 



Upturned strata of the west slope of the Elk Mountains, Colorado. The light-shaded stratum, Tri- 

 assico-Jurassic ; that to the right of it, Carboniferous ; that to the left, Cretaceous. 



Some of the kinds of flexures have been described on pages 93, 94, and views are 

 given of examples from the Green Mountains on page 213, and from the Appalachians 

 on page 396. Another sketch is here introduced, from the Elk Mountains, in Western 

 Colorado, where the hills are free from vegetation, and show their rocks at surface, so 

 that the bendings maj' be easily followed. It represents Cretaceous, Jurassico-Triassic, 

 and Carboniferous strata, and shows that, through a twist in the upturning, the Creta- 

 ceous, which is the overlying rock in the back part of the scene (to the left), is realty the 

 underlying in the front part, and the Carboniferous the upper part, the pressure having 

 so pushed forward the mass that the order of superposition is the reverse of the order 

 of age, and the Carboniferous beds of the front ridge incline 45° beyond the vertical. 

 The flexures and upturning took place after the Lignitic period of the Tertiary; and 

 several of the summits, as measured in 1873 by Gardner, are 12,000 to 14,000 feet in 

 height. The facts and view are from Hayden & Gardner's Report for 1873. 



Geological history is full of such examples and of many of greater 

 complexity. No material is so solid that, when in broad tabular masses, 

 it will not become flexed, by lateral pressure very gradually applied. 

 By " very gradually " should be understood movement by the foot or 

 so a century, or that degree of extreme slowness which has so often 

 been exemplified in geological history, and which is the most common 

 of nature's methods of progress. The rock or other solid, though ap- 

 parently inflexible, will undergo, under such conditions, a molecular 

 movement, adapting it to its new condition. Even brittle ice, as 

 stated on p. 681, becomes flexed by its own weight, if a slab be sup- 

 ported only by its ends. If ice covered a lake to a thickness of a 

 score or more feet, and a slowly-accumulating pressure to a sufficient 

 amount could be brought to bear against one side of it, the ice might 

 be plicated over its surface, as boldly and numerously as the formations 

 of the Appalachians. 



Fractures, Joints. — Fractures are, however, a natural result of the 



1 The prefix in these words is from the Greek for earth ; the bendings are bendings, 

 not of strata or formations, but of the earth's crust covered with its strata, folded or not 

 folded. 



