106 LITHOLOGICAL GEOLOGY. 



synclinal axis is the axial line of the valley, in the plane a x. The 

 word is from aw, together, and kIivu, I incline. 



114. The direction of the strike is ascertained by means of a pocket- 

 compass, and the dip with an instrument called a clinometer. Two 

 instruments of this kind are represented in fig. 102. abed is simply a 

 square block of wood, with, a graduated arc c b, the centre of the arc 

 being at a point near a. From a pivot at this point a plummet or 

 pendulum is hung. On placing the side c d on an inclined plane 

 (A B) the angle is marked off by the position of the pendulum, 

 which of course hangs vertical. 



A clinometer of this kind is often combined with a pocket-compass, the pen- 

 dulum being hung from its centre. This is the most convenient kind of cli- 

 nometer. If there is a black line marking horizontality across the face of a 

 clinometer of this kind, the angle may be taken by holding the instrument 

 between the eye and the dipping edges that are to be measured, and putting 

 this black line parallel with these edges ; the pendulum will mark the angle of 

 dip. In the same manner the slope of the outline of a distant hill or mountain 

 may be measured. 



The other clinometer has the form of a foot-rule jointed at the middle. 

 There is a level at h, by which the leg d e is brought to a horizontal line while 

 the other lies on the inclined plane. The angle between them is read off on a 

 graduated arc near the joint. At i a small compass is attached. 



In using any clinometer, it is well to place a long strip of board upon the 

 layer of rock, lest the unevenness of surface lead to error. 



Faults. — Faults are dislocations of the strata in the plane of a frac- 

 ture, as seen in the coal-layers, figs. 96, 97 ; and the amount of fault 

 is the amount of dislocation. We may say, for example, a fault of 

 ten feet, or one thousand feet, or of five miles, and so on, according 

 to the extent of it as ascertained by actual measurement. 



Fig. 103. 



115. Complexities in stratified deposits arising from denudation and other 

 agencies. — By the denuding action of waters, strata are removed over 



Fig. 104. 



extensive territories, the tops or sides of folds are carried away, and 

 various kinds of sections made of the stratified beds, which are often 

 perplexing to the student. 



