MAPPING OF THE CRYSTALLINE SCHISTS 783 



of the rock, the evidence of crushing, etc. A specimen of the 

 fresh rock, in case the type is new to the region or of doubtful 

 determination, is made by trimming a fragment into the shape 

 of an elongated rectangular pillow three by four inches and of 

 a maximum thickness of an inch. One or more chips for section 

 cutting are also collected. When determinations are extremely 

 difficult a series of smaller specimens untrimmed and showing 

 the weathered as well as the fresh surfaces will be found far 

 more valuable for identification and reference. These to the 

 number of ten or more and representing as different phases as 

 possible may be chipped from a single exposure or from near- 

 lying exposures and given a single number as a series. 



Measurement of strike and dip. — It is a time -honored and 

 almost uniform practice to record the present direction of the 

 plane of bedding in terms of the strike (the direction of the 

 water line if the outcrop were partially submerged) and the dip 

 (its inclination normal to the strike). Both these terms have a 

 definite meaning when applied to the only slightly disturbed 

 sedimentary rocks, and may properly be measured by the text- 

 book methods, the former by leveling the compass with its 

 north-south edge in contact with the bedding plane and reading 

 the bearing, the latter by taking the steepest inclination of the 

 beds — the one normal to the strike. 



In the crystalline schist areas, however, the matter is far 

 from simple. The rock may have no definite structure plane, 

 either because of igneous origin or from metamorphism of a 

 bedded rock. Again it may show one or more prominent 

 structures, but no one of them may represent the plane of sedi- 

 mentation. If one of them is proven to be the plane of bedding 

 the best determination of the strike would probably not be 

 obtained by the text-book method of placing the compass edge 

 in contact with the rock surface. A quicker and generally more 

 correct method is to hold the compass above or beside the 

 exposure and adjust it by the eye to correspond to the average 

 strike of the exposure. The error of the eye in adjusting to 

 parallelism, which should hardly exceed 5 , is inconsiderable 

 when the variation between different parts of an exposure or 

 between near-lying exposures is taken into account. 



