DISLOCATIONS OF STRATA. 



95 



.and character of the upturning. In making observations, see (1) that the outcrop is 

 not that of a bowlder; or (2) of layers displaced by the growing roots of trees or other- 

 wise; also whether (3) the dip and strike are those of merely local or superficial flex- 

 ures, or of the great and general bendings of the rocks. Also consider (4) that, when 

 one fold dies out and another begins at the same time to rise on one side or the other, 

 there will be, as a consequence, transverse strikes over the region between the approxi- 

 mate ends of the two folds. As all folded strata were once horizontal, the study of the 

 flexures of strata is the study of bent or warped surfaces; and the results of observa- 

 tions cannot be right unless they are consistent with one another in this view. 



d. Anticlinal, Synclinal. — In folded strata, the layers bend upward 

 and downward successively ; the upioard is an anticlinal flexure (from 

 avri, opposite, and k\lvw, I incline), and the downward a synclinal (from 

 avv, together, and kAiVw). In the anticlinal (Fig. 99, A, c, D, and 

 either summit of b), ax is the anticlinal axis, or that away from 

 which the layers slope ; and in the synclinal (middle part of Fig. 

 99 b), a' x r is the synclinal axis, toward which the layers slope. In 

 Fig. 100, a, a mark the positions of two anticlinal axes, and a', a' those 

 of two synclinal axes. The roofs of ordinary houses are examples of 

 auticlinals, and the ridgepole has the direction of the anticlinal axis. 

 If the ridge-line of a fold is inclined, then the anticlinal axis is said to 

 be inclined. In a monoclinal ridge, the beds all dip in one direction. 



The direction of the strike and the dip are ascertained by means 

 of an instrument called a clinometer, which is in part a pocket compass. 

 A common kind is a pocket compass of the size of a watch, having 

 a pendulum at centre to note by its position the angle of dip. The 

 best has a diameter of 3£ inches, and a square base whose sides are 

 parallel to the principal diameters of the circle. The part of Fig. 

 102 to the right illustrates the use of the pendulum, and shows how a 



Fig. 102. 



cheap form of clinometer may be made. On placing the side c d on 

 an inclined plane (A B), the angle is marked by the position of the 

 pendulum, which of course hangs vertical. Another kind of clinometer 

 is shown in the upper part of the same figure. 



