TERRANES. 



101 



the dip may be obtained thus : take the dip and the direction along two of the sections ; 

 then, from a point, A, draw two straiglit lines, AB, AC, in the,directions of the observed 

 dips, and set off, on each, lengths proportional to the cotangent of its own dip, Ab, Ac ; 

 then, a line through b, c will have-the direction of the strike, and a perpendicular to it, 

 that of the dip. 



In studying a region of rocks it is important that the dip and strike should be obtained 

 at all outcrops, and noted down on a map. Eor the latter, the best mode is to use a 

 symbol like the letter T, giving the top the direction of the strike and the stem that of 

 the dip ; and the different angles of dip may be approximately indicated by variations in 



90. 



[ hh 



90= 



80° 70° 60° 50° 45° 35= 



25= 



15° 



the length of the stem of the T, as in the annexed figure, in which the ratio of the stem 

 to half the top of the T is for 80° = 1 : 4 ; for 70°, 1:3; for 60°, 1:2; for 50°, 1 : li ; for 

 45°, 1:1; for 35°, 11; 1 ; for 25°, 1|: 1 ; for 15°, 2:1; and for horizontality, a crossed 

 circle. 



Flexures. — Some of the forms of flexures are illustrated in the following 

 figures. Such flexures, while often very small, may be several thousands 

 of feet in height, and many are miles in span. The following are a few of 

 the forms. The slopes either side of the center are seldom equal. In 

 4; Fig. 91, Aa is the axis of the flexure, and in both of those to the right 



91, 



this axis of symmetry is inclined ; and in 5 and 6, still more inclined ; 

 while in 7, 8 (from the Alps) other complexities are represented. Flexures 

 like those in the right half of 5 and in 6 are called overthrust flexures, 

 the flexing being due to pressure from the right. Supposing the pressure 



