THE PRECAMBRIAN ROCKS OF THE CANTON QUADRANGLE 97 



the photograph accordingly facihtates visualizing an uncommon 

 type of structure whose dimensions alone prevent its easy recogni- 

 tion in the field. 



This huge fold, whose outcropping edge, as mapped (figure 21), 

 describes the doubly reflexed curve occupying the whole south- 

 eastern corner of the sheet, is a tightly compressed S-fold, whose 

 pitch, generally about 25 to 30 degrees, is directed nearly at right 

 angles to the strike, that is, perpendicular to the average trend of the 

 formations. The usual direction of the latter on the northwest border 

 of the Adirondacks, and indeed throughout the whole extent of these 

 mountains, is very decidedly in a northeast-southwest direction, and 

 in this respect the formations within the present quadrangle are no 

 exception. Locally, however, on account of folding, there is de- 

 flection, or even complete reversal through an arc of 180 degrees, 

 with the result that in the case of large folds involving thick for- 

 mations there are considerable areas, as about the points of reflec- 

 tion, where the strike trends toward the northwest, nearly parallel to 

 the direction of the pitch. The greatest dimension of the Pierrepont 

 fold, measured in an east-west direction, is over 6 miles ; measured 

 parallel to the strike of its axial planes, it is 5^ miles; at right 

 angles to this, or across the breadth, it is nearly 5. It has involved 

 in its contortions not less than 36CX:) feet of gneisses and schists 

 which have been completely doubled back upon themselves at two 

 points, in opposite directions respectively. The rocks in cpiestion are 

 limestone, quartz schist, garnet gneiss, hornblende schist and granite, 

 together with the composite injection zones produced by the inter- 

 mingling of the latter three. 



No thinning of the middle limb, nor thickening at the points of 

 reflexion, such as is common among tight folds with horizontal 

 axes, is to be observed; the strata maintain an approximately uni- 

 form thickness throughout their entire course. It seems most prob- 

 able, however, that the limestone, which occupies the inner portion 

 of the right half of the fold, after being doubled back upon itself, 

 was then greatly thinned by a compression transverse to its present 

 strike. The associated belt of quartz-mesh limestone and quartz 

 schist undoubtedly took part in this process, but the outcropping 

 portions do not show the expected reversal of strike. It is possible 

 that this fact is to be accounted for by an extraordinarily acute com- 

 pression of the innermost part of the isocline, like that of the zig- 

 zag xenolith of garnet gneiss south of the Waterman Hill drumlin. 

 In this case, unless the actual apex of the fold were exposed, the 



