ROGERS, GEOLOGY OF THE CORTLANDT SERIES 55 



found in the granites, norites or pyroxenites, although at the same time 

 it is not severe and is apparent in the hand specimen only in exceptional 

 cases. The gabbros in two cases show a remarkable degree of crushing, 

 such as would be characteristic of an augen gneiss, and such as was de- 

 scribed by Lehman 40 in the "Flaser Gabbros" of Saxony. 



Both of these cases, however, occur directly on the border of an inclu- 

 sion : and those of the diorites which show a gneissoid structure are simi- 

 larly located. Williams has described the crushing which is apparent in 

 some of the types found in the "Butler Section"; it is here also very 

 severe microscopically, but this section is located 150 yards west of the 

 inclusion which runs northwest from near Montrose. In many of these 

 instances, it is easy to take the strike and dip, but the cases of straining 

 which were noticed elsewhere were relatively insignificant. It would thus 

 seem to appear that dynamic metamorphism is usually or always confined 

 to the borders of foreign inclusions; and if the district had undergone 

 any amount of regional pressure, it would naturally be concentrated 

 along the lines of weakness which would develop on the contact. This 

 is the only explanation that would seem to account for such a localization 

 of metamorphic effects. 



It may be noted in this connection that dynamic action is also very 

 perceptible at most of the emery developments in the district; it is evi- 

 denced not only by microscopic and megascopic shearing, but by faulting 

 and veining. 



ORIGINAL GNEISSOID STRUCTURE 



Dana 41 describes in some detail a structure which the present writer 

 believes to be of an original gneissoid character. As already stated, on 

 Montrose Point, in particular, several very different kinds of rocks are 

 associated in the most intricate way, often as successive bands; 42 and 

 Dana cites one case in which biotite augite norite and olivine pyroxenite 

 are found in alternate layers of constant grain only three or four inches 

 wide. There are many other cases of less pronounced character, which 

 are referred to by both Dana and Williams (e. g., the Butler Section) 

 and which have been seen by the writer. In another instance, a streak 

 of the coarse dark pink norite was seen in a cliff of the black pyroxenite ; 

 the norite was coarser, if anything, than the latter, and was coarse, more- 

 over, to its very edge, having thus none of the characteristics of a dike. 

 The analyses of these types show their great chemical differences. 



40 Uber die Entsthehung der altkrystallinen Schiefergesteine, p. 190. Bonn, 1884. 

 « Amer. Jour. Sci., (3), XX, 211 et seq. 1880. 



42 Williams refers to these as "dikes," but Dana evidently recognized that they were 

 not of this character. 



