232 Pogue, Jr. — Geology and Structure of Volcanic Rocks. 



It is probable, also, that the major folds are not absolutely 

 horizontal, but pitch slightly, so their crests form wavy lines. 

 No direct measurements of pitch can be obtained, but infer- 

 ence as to its nature may be made from the way in which 

 certain formations end abruptly against others, as if dipping 

 beneath them. A further evidence is the occasional discord- 

 ance between trend of bedding and of schistosity, indicative of 

 a complexly folded region.* 



In addition to these major and minor directions of folding, 

 whose axes lie in a horizontal plane, the whole region has, 

 perhaps, been slightly bent around a vertical axis. A glimpse 

 at the geologic map will disclose the tendency of the schistosity 

 to form an arc-like arrangement ; in general, varying from a 

 northeast trend near the river to a more northerly direction as 

 the upper limits of the map are approached. 



Mashing. — It has been suggested in the previous section 

 how schistosity has been induced upon much of the region by 

 the same compression which occasioned folding, and how this 

 has been more prominently developed on the limbs of the 

 folds than on the crests or in the troughs. A further deduc- 

 tion, however, may be made from the nature of the schistosity ; 

 that is, the direction along which the compressive force acted. 

 The average trend of schistosity is N. 50° E. Theoretically, 

 therefore, the compression acted along a line passing N. 40° "W., 

 as the effects of compression are at right angles to the force. f 

 This figure must not be taken as exact, for other factors would 

 complicate the result ; but it is approximately true. 



The average dip of the schistosity is about 70° to 80° to the 

 northwest, with extremely few cleavage planes dipping to the 

 southeast. This is significant. It suggests that the folds are 

 not upright ; for in such a case approximately half of the 

 schistosity should have a dip to the southeast. Thus there is 

 evidence for believing that the folds are slightly inclined, 

 their axial planes agreeing in a general way with the average 

 dip of the schistosity. This view is consistent with the arc-like 

 arrangement of the formations, which in itself implies a slight 

 overriding of the upper crust and an axial dipping of the folds 

 towards the center of the arc. 



The northwest dip of cleavage planes and northwest-facing 

 concavity of the arcs is the opposite of conditions holding in 

 the Appalachians. This may be due either to some undeter- 

 mined local cause, or to an actual reversal of the relations 

 between land and sea obtaining in the Paleozoic ; so that in 



* Van Hise, C. R., Principles of North American Pre-Cambrian Geology. 

 16th Ann. Kept. U. S. Geol. Survey, 1895, p. 629-630. 



f See in this connection Haug, Emile, Traite de Geologie, vol. i, p. 227, 

 1907. 



