100 



STRUCTURAL GEOLOGY. 



is in the opposite direction from the dip. The stj-ike is the liorizoutal direction, 



St (Fig. 87), at right angles to the dip. The direction of strike is ascertained 



by means of a compass ; and 

 the angle of dip by a clinometer. 

 A cUnometer-comjxiss is a com- 

 pass in which the movement of 

 a plummet measures the angle 

 of dip, the degrees being marked 

 on a graduated arc, as shown in 

 Fig. 89. The compass in its best 

 form has a square base, with one 

 side of the square parallel to 

 the N.-S. line, so that the side 



may be used in place of the line, or the instrument may be applied by one 



side to the rock, or used in sighting distant slopes. 



The edges of outcropping layers give the true clip only when the section affording 

 them has the direction of the dip, as those on the right side of Fig. 87, or those of the 

 side 1 in Pig. 88 ; but those of sides (or sections) 2 and 3 in the latter figure vary in direc- 

 tion from the dip ; and those of 4 have no dip, but are horizontal, and have therefore the 

 direction of the strike. 



In the best clinometer-compass the square base is about 3 inches in diameter. A 

 clinometer (Fig. 89 B) is easily made out of a block of hard wood, 3 to 3t inches square, and 

 half an inch thick. A small compass may be set into the same block, with its N.-S. line 

 parallel to one side of the block, as in the figure, making the instrument serviceable for 

 taking directions of strike or dip, though too small for good compass work. 



In making observations, first take the strike, and in recording it refer it to the north 

 point ; e.g. N. 20° E. (if that be the direction), never S. 20° W. ; only the direction of 

 glacial scratches should be referred to the south point. Next note whether the dip is 

 easterly or westerly, and measure the amount ; if 50° easterly, then it is 50° in the direc- 

 tion S. 70° E., this course being at right angles to the strike. The entry "strike N. 20° E., 

 dip 50° E." includes the whole. To obtain the true strike, the edge of the laminated rock 

 selected for the measurement must be perfectly horizontal ; if there is none such in an 

 outcrop, draw a horizontal line on one of the beds. The error from a variation from 

 horizontality increases as the dip decreases, and becomes null only when the dip is 

 vertical. 



In taking the strike, the side of the square compass parallel to its N.-S. line should 

 be used ; and it is better to apply it to a piece of board laid over the rock than to the 

 rock itself. But it is not necessary to put it on the rock ; it is generally best to make the 

 observation standing, with the N.-S. side of the compass between the eye and the out- 

 cropping edge. The same method may be used also in taking the dip ; and if the observer 

 is in the line of strike, he can thus take the dip even when the ledge is rods distant. The 

 slope also of a mountain on the horizon can be obtained with a clinometer in the same 

 way. 



Before making a measurement, it must be ascertained that the outcrop is not that of 

 a bowlder, or of layers displaced by the growing roots of trees ; and that the particular 

 locality will give a mean, not a local, result. Perfectly uniform strikes or dips for a 

 distance of a hundred yards are not generally to be expected, — a fact that will trouble the 

 young geologist in his first field observations. 



When, among the exposed sections at a place, none is at right angles to the strike. 



