l->t! G. 8. Rogers- Original Gneissoid Structure 



through Stony Point, he abandonedjiis former explanation and 

 pronounced the rocks truly igneous. In 1886 Professor G. II. 

 Williams* published his first paper on the Series, and in it 

 treats the rocks as unquestionably igneous, lie also mentions 

 their occasional streaked appearance, and points out that this 

 is just what would be expected in igneous rocks which had 

 undergone some regional metamorphism. 



The present paper is therefore not the first to describe the 

 structure, although it is the first to interpret it as originally 

 igneous. Pecognizing the existence of such an original gneiss- 

 oid structure in other parts of the world, it becomes evident 

 that Professor Dana has described several rather obvious exam- 

 ples of it. The northern part of Montrose Point, where these 

 cases occur, is a complicated mixture of a biotite-augite nor- 

 ite f and an olivine pyroxenite, the latter lying mainly to the 

 south and west. These varieties appear to interpenetrate very 

 intricately, and occasionally a structure such as Professor 

 Dana describes is to be found. The writer noticed in one case 

 a streak, about four feet wide, of the coarse dark pink norite 

 in a cliff of black pyroxenite. This comparatively narrow 

 strip was coarser, if anything, than the pyroxenite, and was 

 coarse moreover to its very edge, having thus none of the char- 

 acteristics of a dike. Analyses of these two types, given 

 below, show their great chemical differences. 



It is in the various norites, however, that the structure is 

 best shown. It may be stated first, as a general rule, that the 

 finer grained a norite is, the simpler it is, i.e., a very fine- 

 grained norite is composed chiefly of feldspar, with consider- 

 able hypersthene, while the coarser varieties carry in addition 

 either hornblende or biotite and augite. The fine-grained 

 simple norite is never found in large areas, but always as inclu- 

 sions in the coarser and therefore more complex varieties. 

 It often occurs in biotite norite, for example, as small, 

 rounded flow-like patches, or again as streaks ; or it may be 

 banded with the coarser rock. In this case the chemical dif- 

 ference is not so great, as the accompanying analyses show 



A. Biotite augite norite, from Montrose Point. Analysis by 

 M. D. Muim, for J. D. Dana, this Journal (3), xxii, p. 104, 



* The principal papers are : Peridotites of the Cortlandt Series, this Jour- 

 nal (3), xxxi, 26 ; Norites of the Cortlandt Series, idem (3), xxxiii, p. 

 l3o and p. 191 ; and Gahbros and Diorites of the Cortlandt Series, idem (3), 

 xxxv, p. 438. 



f The names used to denote the different rocks are, it is believed, suffi- 

 ciently explicit to obviate, for the purposes of this paper, a more detailed 

 petrographic description . The analyses given serve the purpose; Professor 

 Williams, in the papers cited above, give3 very minute descriptions should 

 these be desired. 



