784 STUDIES FOR STUDENTS 



Something further will be said upon the differentiation of 

 original and secondary structures 



Important additional observations. — Pitch and its measure- 

 ment. — Hardly less important than the inclination of the bedding 

 plane at a locality is the pitch of the folds — the inclination of 

 their trough and crest lines. Vital as is this element in the 

 determination of the structure of an area, it is only in recent 

 years that it has been given much weight in field investigations. 

 It is safe to say that its importance is yet only half appreciated. 

 To Professor Raphael Pumpelly, who directed the early field 

 work in the New England area, is due the credit of bringing to 

 the front this element in the field study of crystalline rocks. 

 The details regarding the methods of measuring it were worked 

 out by the assistants in his division. 1 



The pitch is often consciously or unconsciously measured as 

 a component element of the dip, the dip being increasingly 

 determined by the pitch as the outcrop is near the crest or the 

 trough of a fold. Pitch can only be measured at the locality 

 when the outcrop exhibits minor folds or plications, the study 

 of the Green Mountains having clearly demonstrated the fact 

 that the minor fold is the epitome of the major fold. The pitch 

 is subject to the same kind of variations within an exposure as 

 is the dip, and it must not be recorded from a single minor fold 

 or plication without first noting whether other folds or plications 

 give similar values. Even then its inclination cannot fairly be 

 assumed beyond the exposure itself, and it is only by examina- 

 tion of a series of exposures that a persistent pitch is determined. 



When not to be made out at an exposure, pitch (and here a 

 persistent one) is correctly inferred when the strikes of the 

 opposite limbs of a fold are other than parallel. A syncline 

 pitches in the direction in which the strikes of its opposite limbs 

 diverge and an anticline in the direction in which they converge. 



Inferences regarding the pitch may often be drawn from the 

 profiles of ridges, the gradually sloping lines being formed by 

 the crests of folds. 



'See Monograph XXIII, U. S. Geol. Surv.; also Van Hise, "Principles of Pre- 

 Cambrian Geology," Sixteenth Ann. Report U. S. Geol. Surv., Part I, 1896, pp. 603-632. 



