MAPPING OF THE CRYSTALLINE SCHISTS 787 



limbs converge downward in an anticline and upward in a syn- 

 cline. z 



Joint planes. — Joint planes more or less perfectly developed 

 are almost universally present in the crystalline rocks. They 

 are planes of separation in parallel series, of which more than 

 one is usually to be made out. They usually have a very steep 

 inclination and are probably like the folding, in most cases a 

 result of the compression of the area. Their number (z. e., the 

 number of series) and relative perfection, and the direction and 

 inclination of their walls, should be carefully noted, with the 

 relation of these to lines of displacement and to topographic 

 features. 



Scarps a?id steep rock walls. — Little attention seems to have 

 been accorded these features of rock exposures. In the sequel 

 I propose to show their importance in the geological structure of 

 a region. Their direction and magnitude should be noted, and if 

 possible some symbol easily distinguished from the dip and 

 strike symbol should be entered at least upon the working map. 

 A short dotted line will serve to indicate the direction of the 

 scarps and a figure at its end its bearing to the east or west of 

 north. 



Margi?i of outcrop. — Another neglected feature of rock expo- 

 sures is the margin of the outcropping. Every experienced 

 geologist must be able to recall numerous instances where out- 

 crops, grouped so thickly as to indicate probably but slight 

 covering in the intervening area, cease abruptly at a border 

 which is often nearly or quite rectilinear and perhaps is also for 

 sections of its extension the margin of individual exposures. 

 Such observations, it is believed, are of great significance in the 

 proper interpretation of the structure of a region. 



Gorges and sharp straight valleys. — Even the best of the topo- 

 graphical maps do not indicate all gorges or rectilinear valleys 

 with rock walls. In the interpretation of the geological struc- 

 ture their significance may be considerable, and it is important 

 to note their direction and in many cases also the height of their 



1 See on the general subject of foliation, Pumpelly, Wolff and Dale, Mono- 

 graph XXIII; U. S. Geol. Surv., pp. 136-158, Pt. I, 1896; and Van Hise, Six- 

 teenth Ann. Report U. S. Geol. Surv., Tt. I, pp. 633-668. 



