106 



STRUCTURAL GEOLOGY. 



by another, very much like the folds above the elbow in a woolen coat- 

 sleeve. The flexures are those of a warped surface, parallel usually in 

 direction, but mutually involved along their course. Hence there are large 

 variations in dip between the flexures. 



105. 



Stf JO^TaHCXMAW. 



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

 Jurassic; that to the right of it, Carboniferous; that to the left, Cretaceous. Holmes, Gardner. 



Models of flexures may be conveniently made out of a large mahewn branch of a tree 

 of coarse-grained wood, having the bark on. A piece of the branch (3 or 4 inches or 

 more in diameter) 12 to 18 inches long, cut obliquely from a diametral line at one end 

 at an angle of 20° or so, will afford two models of a flexure with an inclined axis. By 

 coloring groups of layers in the wood, using for greater simplicity not more than three 

 colors, the appearances of the flexed strata may be studied in horizontal, vertical, and 

 any other sections that may be cut. Such models might be made by pasting together 

 sheets of differently colored paper, or layers of paper-pulp, and so making a cylinder, and 

 then cutting it as above. By pressure the cylinder might be made elliptical, and models 

 might be obtained with unequal dips on the two sides. 



Geanticlines, geosyndines. — The flexures in rocks which have been above 

 described and illustrated by figures are flexures of the strata of the earth's 

 exterior, or the supercntst, not of the crust itself. The crust is thick, and 

 it is impossible, were it but 10 miles thick, that it should be bent into 

 so small and abrupt flexures. It has, however, its own great flexures of low 

 angle and of great breadth, both upward and downward. It is proved that 

 the stratified rocks of the Alleghanies were laid down in one such downward 

 bend or trough, a thousand miles long, during the long ages in which it was 

 slowly deepening. There are also evidences that upward bends of similar 

 extent have been made. These flexures of the crust are termed geanticlines 

 and geosyndines, the prefix in these terms being derived from the Greek 

 word for earth. The basin of Lake Superior probably corresponds to a 

 geosyncline, as suggested by T. C. Chamberlin. 



Fractures, faults, compression and stretching of rocks. — The fractures 

 intersecting rocks are of all sizes, from those small cracks that result from 

 contraction on drying and cooling, and from gravitational pressure on strata 

 of varying compressibility or of insnflicient support, to those, sometimes 

 miles in depth, that are made in the grander movements of the earth's 

 crust. 



