30 ANNALS NEW YORK ACADEMY OF SCIENCES 



petrographical basis is not academic is evidenced by the areal distribu- 

 tion — the various types fall naturally into distinct areas. Their general 

 distribution is suggestive, for although occasional isolated patches occur, 

 practically the whole of the norite magma, in its various ramifications, is 

 confined to the central part of the Cortlandt area, being flanked on the 

 west by diorites and pyroxenites and on the east by the latter. At the 

 north and south, they are often separated from the mica schist by sye- 

 nites, diorites and gabbro ; and if the boundary could be accurately traced,, 

 it is very possible that these or similar rocks would be found continuously 

 along the margin. 



True norite, which is composed merely of hypersthene and feldspar, 

 occupies a very peculiar position in the series. It does not appear on the 

 map, since it never covers an appreciable area ; it occurs always as inclu- 

 sions in the other members of the norite family and generally, if not 

 always, in those containing biotite. These inclusions are sometimes 

 streaks and sometimes rounded flow-like patches, and this imparts a very 

 peculiar appearance to the norites, especially in the northwestern corner 

 of the area. The inclusions vary in size from a few inches to possibly 

 forty or fifty feet across, though the smaller ones are the most common. 

 Absolutely pure specimens are rare, inclusions in which augite and biotite 

 appear in insignificant amount being the general thing. This pure norite 

 is found frequently in the district between Spitzenberg Hill and Peeks- 

 kill, especially along the post-road to the west ; at Pleasantside again it is 

 well developed, and striking cases were also found (see Plate IV, fig. 2) 

 in the woods a mile south of Spitzenberg. It may occur anywhere in the 

 biotite-bearing norites, however, and the above cases are merely a few 

 examples. 



The rock is easy to distinguish in the field, since it is always fine 

 grained. Its color is usually pale pink, owing to the amount of plagio- 

 clase present, and scattered through this are the black hypersthenes. As 

 the grain grows coarser, a little biotite and augite may enter, and the 

 rock conceivably grades into the more complex and coarser norites. 



Under the microscope, it appears that the pink color of the plagioclase 

 is due to the presence of a fine reddish dust, which Williams has figured 

 and discussed at some length. 28 Under a high power, this dust resolves 

 itself into plates, rods and globulites, presumably of hematite. The 

 plates may be hexagonal, rectangular or irregular, and they vary in size 

 up to .01 by .0-4 mm. The rods are often arranged in rows, forming a 

 kind of discontinuous needle, but these exhibit no parallelism to any 



* Amer. Jour. Sci.. (3). XXXIII, 141. 1887. 



