322 C. R. KEYES SECTION ACROSS THE PIEDMONT PLATEAU. 



mechanical deformation, the edges and angles being ground away and the 

 fragments still filling the interstices. The larger quartz grains exhibit, 

 between crossed Nicols, marked undulatory extinction — a ph nomenon 

 quite characteristic of granitic masses that have been subjected to great 

 dynamic action. 



Considerable interest attaches to the structure of Sugarloaf mountain, 

 which was incidentally a subject of consideration in the construction of the 

 section across the plateau proper. The thick massive sandstone forms a 

 monoclinal with easterly dip. As regards the somewhat lower elevation to 

 the west of the mountain, two hypotheses are presented, either of which 

 would offer a satisfactory explanation ; but the substantiation of one or the 

 other is immaterial in the present connection. There may have been a 

 double thrust, thus giving the sandstone a measurement twice as great as 

 the actual thickness ; or the crests of the two elevations may represent 

 parts of the same formation in which only a small amount of sliding move- 

 ment has taken place. The former of these suggestions, however, appears 

 the more probable. 



As already stated, the upper portion of the sandstone passes gradually, by 

 intercalation of thin argillaceous bands, into the schists lying to the eastward. 

 These schists, for a considerable distance from the mountain, show no ap- 

 parent contortion. Thin cleavage planes are coincident with the dip of the 

 Sugarloaf sandstone. The regular succession of numerous thin argillaceous 

 and sandy layers above the massive portion of the great sandstone would, 

 therefore, seem to indicate that the cleavage directions of these rocks are true 

 planes of stratification. In some of these undisturbed transition beds to- 

 ward the superior limit of the great sandstone the alternation of different 

 lithological materials is so marked that layers of sandrock, in every respect 

 identical with the Sugarloaf stratum, and only from four to twelve inches in 

 thickness, regularly succeed equally thin seams of fine clayey sediments. 

 In places the effect of the light buff color of the narrow sandstone bands 

 and the dark blue-black layers of the argillites is very striking. 



The slates on the western side of the Sugarloaf prominence are variously 

 inclined, from nearly perpendicular to a comparatively low angle. They 

 appear in places considerably puckered. 



When the region was subjected to intense orographic pressure, the softer 

 rocks were finely crinkled and puckered. On the other hand, the great 

 thickness of sandstone was but little affected internally. It was faulted and, 

 acting in large units, apparently slid over the softer layers. The facts as 

 here presented show that below the sandstone the shales are more or less dis- 

 turbed ; while above, the argillites are not at all affected, and appear inter- 

 stratified with the upper portions of the arenaceous formation. This would 

 point strongly to the conclusion that the plane of movement or thrust was 

 at the bottom rather than the top of the great Sugarloaf sandstone. 



