THE PRECAMBRIAN ROCKS OF THE CANTON QUADRANGLE IO3 



The possible subsequent history of such a fold is indicated in fig- 

 ures 26-30 in such a manner as to show that the Pierrepont sigmoid 

 (plate 18, upper figure and figure 31) is one of a series of positions 

 which an isocline of figure 25 type might assume during the process 

 of deep-seated deformation incident upon a change in direction of 

 the active forces or on account of a torsional element present in 

 those forces. In these stages tilting, compression or some other 

 adequate force, acting presumably from the northeast, is called upon 

 to tilt the horizontal axes of figure 25 more and more in a southwest 

 direction, till they finally become revolved through a quadrant in 

 the axial plane. It w411 be noted that while the pitch or axis of the 

 fold is as nearly parallel as possible to the strike of the axial plane 

 in figure 25, yet as the fold is tilted more and more steeply to the 

 southwest, the azimuth of the pitch swings round progressively, 

 becoming more and more nearly parallel to "the direction of dip of 

 this plane; till at the final stage (figure 29) where the axis has been 

 revolved a full quadrant, the pitch is parallel to the regional dip of 

 the foliation, that is, to the dip of the axial plane. In the latter 

 case, the conditions are practically the same as in the C-fold on the 

 Island of Uto, southeast of Stockholm, except that there the axial 

 plane is vertical (Holmquist, 1910, pages 104-6). 



In figure 29 the conditions of the Pierrepont sigmoid are repre- 

 sented ; the axis, formerly horizontal, is tilted southwest almost 

 through a quadrant in the axial plane (lacking in fact about 15 

 to 20 degrees of it), with the result that its azimuth approximates 

 that of the regional dip w^ithin the angle mientioned. Throughout 

 this paper, regional dip has been tised to signify the average dip 

 of the formations considered through the extent of a complete 

 sigmoidal fold, or the dip of the isocHnal limbs, or in other 

 words it is the dip of the axial plane. Various other types of 

 isoclinal folding in Precambrian formations can be referred to one 

 or another of these stages, but the figures for this purpose should 

 be elaborated to include the vertical and horizontal as well as the 

 inclined positions of the axial plane. 



A structure analogous to the large compressed folds of the Can- 

 ton quadrangle is described as occurring in the body of zinc ore at 

 Franklin Furnace, New Jersey. (See Spencer et al., 1908, 

 pages 24-25, figures 11-15; Nason, 1895, page 127, figure 4.) 

 The simiHtude ceases, however, with the isoclinal character of 

 the latter. The fold is not sigmoidal nor is the angular relation 

 between the pitch and the strike the same. In the Franklin Fur- 

 nace fold the pitch departs only a few degrees from the strike, with 



