I06 NEW YORK STATE MUSEUM 



of the Open fold, one finds on following the formation from limb to 

 limb, as at a and h, the strike changes through nearly i8o degrees, 

 and, the dip, after swinging round through an equal angle, is then 

 tow^ard the opposition direction. At a and h, the points of reflection, 

 the dip of the bed is parallel to the pitch of the fold. In figure 25, 

 and in the following figures where the folds are overturned, there 

 are certain points (as 0, 0) where the bed becomes vertical on 

 tracing it round the outcrop ; and the proximity of these points to 

 the actual points of reflection increases with approaching coin- 

 cidence of the pitch with the dip of the axial plane. In this latter 

 event, as in figure 30, the points of flexure and the points of verti- 

 cality of the beds are identical. In the Pierrepont fold, allowing 

 for slight irregularities in the application of the rule, there is a 

 marked tendency for the formations to become vertical northwest 

 of, rather than at, the points of reflection. After passing these 

 points of verticality the dip diminishes gradually, and finally assumes 

 its original direction. It thus appears that the trace of outcrop- 

 ping beds may have approximately the same curvature on a map, 

 and yet may represent the beveling of totally different folds of 

 open, asymmetrical or isoclinal character, with either horizontal or 

 steeply inclined axes. The tectonic significance of these possibilities 

 is apparent. The shape of the curvature is only roughly similar, 

 however, for as the pitch steepens, its surface elongation in the 

 direction of the parallel limbs becomes less and less ; and it is at its 

 minimum in figure 30, where the pitch is in the same direction as 

 the dip of the axial plane. 



It is an interesting fact that the term pitch has been applied 

 by geologists to two widely different structures, but in this paper 

 no distinction has been attempted, for the reason that intense and 

 prolonged metamorphism and deformation have actually rendered 

 the two coincident in direction. As originally applied by Cook, state 

 geologist of New Jersey, it referred to the direction of elongation 

 of the pods or pencil-like bodies of ore, and of mineral streaks and 

 aggregates in the gneisses of the New Jersey highlands. In Appala- 

 chian and Alpine geology, on the other hand, the term has been ap- 

 plied generally to the actual axes of open, asymmetrical and reversed 

 folds, to the direction of these axes and to their angular departure 

 from the plane of the horizon. From the fact that in the Canton quad- 

 rangle the elongation of mineral streaks in the gneisses and schists 

 and the direction of the axes of major and minor folds coincide over 

 a large area, it is believed that the term pitch is advisedly though 



