I02 



NEW YORK STATE MUSEUM 



Structure illustrated in the Pierrepont sigmoid. The first three fig- 

 ures (23-25) are designed to show the hypothetical passage of an 

 open fold of the Appalachian type through an asymmetrical stage to 

 a tight isocline like those common in the Alps, the axes in all cases 

 remaining horizontal or nearly so. The usual feature of approximate 

 paralleHsm of the pitch to the strike of the limbs therefore holds good 

 without question through these stages of deformation in which the 

 compression has acted constantly in one, the northwest-southeast, 

 direction. In folds of this kind the direction, or in a surveyor's 



Fig. 31 Sterogram of the Pierrepont sigmoid, viewed from the south. 

 Compare plate 18, upper figure and figure 29. 



sense, the azimuth of the pitch varies but a few degrees from the 

 average trend of the formations, on account of the fact that the 

 angular departure of these axes from the horizontal plane is slight. 

 This relationship, moreover, holds true for any dip of the axial 

 plane, except the vertical position, in which case, and in this alone, 

 the pitch and average strike (that is, th^ axial plane) coincide in 

 azimuth. The first three figures thus illustrate an early stage of de- 

 formation in this region, in which the forces have presumably acted 

 in a northwest-southeast direction. 



