44 NEW YORK STATE MUSEUM 



invariable presence in one or both walls of the magnetite bodies is 

 a feature that commands attention and that has been remarked also 

 by Spencer in his work in the New Jersey magnetite districts. The 

 pegmatites are only coarser phases of the normal granites and like 

 them probably are of different ages. 



Aside from granite local intrusions of syenitic and dioritic rocks 

 occur in the region. At the Croton mine near Brewster the ore 

 is associated with an augite quartz syenite of greenish color, quite 

 analogous to the augite syenites of the Adirondacks. The last series 

 of intrusions is represented by traps, which occur in small dikes 

 and intersect all the crystalline formations. 



Crystalline limestone with interbedded graphitic and pyritic 

 quartzites and mica schists is found in belts that parallel those of 

 the gneisses. The series is very similar to the Grenville limestones 

 and schists of the Adirondacks and, like the latter, has been in- 

 truded and metamorphosed by the igneous rocks. Such contact 

 phases carry secondary silicate minerals in greater or less abundance. 



The present attitude of the formations is monoclinal — the strike 

 being uniformly northeast and southwest and the dip at a high angle, 

 usually to the northwest. Swings of a few degrees occur in the 

 strike from place to place, and both strike and dip are subject to 

 local undulations as shown particularly in the mine workings. 

 Southeasterly dips have been noted at some of the mines. The ore 

 bodies themselves usually show also a pitch by which the longer 

 axis of the pods and lenses as well as of the minor undulations 

 trend downward toward the northeast. 



The magnetite deposits are not limited to any one of the forma- 

 tions which have been named nor to any single set of conditions 

 with respect to them, but have various relations, as will be shown. 

 In composition, they range from lean concentrating ores to those of 

 shipping grade carrying from 55 to 65 per cent iron. They include 

 both pyritous and low-sulphur magnetites and are mostly o\ rather 

 fine grain. 



Jn perhaps the greater number of localities the ore is associated 

 with the banded gneiss and forms a layer or lens within the latter, 

 itself showing a banded arrangement on the borders by alterations 

 of magnetite and the gneiss. 'Hie ore bodies conform to the struc- 

 ture of the gneiss in every particular, following even the minor 

 undulations of dip and strike. They are comparatively thin, seldom 

 Over 8 or 10 feel across the dip, but on the other hand very per- 

 sistent along the outcrop and apparently also in depth so far as 



they have been tested. A feature o\ their occurrence in many 



