MINERAL RESOURCES OF THE STATE OF NEW YORK 1 7 



of ridges, attaining true mountain proportions in the northern part, 

 that enter the State from New Jersey and continue northeasterly 

 into New England. Their summits are mostly under 2000 feet but 

 they are often very rugged and consequently given up mainly to 

 forest. The ridges die out rapidly on the south side, and in 

 Westchester county the surface is only moderately broken although 

 the crystalline rocks extend over practically the whole area and 

 southward into Manhattan island. 



The geology of this region is very complex and much of it still 

 awaits study. Precambrian gneisses are a large element of the 

 crystalline formations; they comprise some igneous types like the 

 Storm King granite-gneiss and also banded gneisses that possibly 

 belong in the Grenville series. With the latter are associated small 

 belts of limestone and quartzite. There are gneisses of syenitic 

 tjrpe which resemble the Adirondack syenite, and some of more 

 basic character, but anorthosite apparently is absent. The igneous 

 intrusions continued as late at least as the early part of Paleozoic 

 time; the Cortlandt series of highly differentiated rocks ranging 

 from granite to gabbro and pyroxenite, in the vicinity of Peekskill, 

 is possibly of Silurian age. 



The principal ores in this region are magnetites which are asso- 

 ciated with the Precambrian gneisses, occurring in belts arranged 

 along northeast-southwest axes, parallel with the general structural 

 trend. A banded gneiss frequently outcrops in the vicinity of the bodies, 

 but this is always injected with granitic material, and granite or 

 pegmatite may usually be found to constitute one wall of the ore. In 

 one or two of the deposits the ore is found on the contact of granite 

 and limestone and has a gangue of secondary silicates. Deposits 

 of magmatic type are represented by the magnetites in the vicinity of 

 Brewster which have a syenitic gneiss for the country rock. Bodies 

 of impure magnetite containing some titanitmi are present in the 

 Cortlandt gabbros. Corundtmi is a sporadic ingredient of these 

 ores and its appearance renders the material useful for abrasive 

 purposes, forming a low-grade emery. Arsenopyrite and pyrrhotite 

 are fotind in one or two localities in the Highlands gneiss belt and 

 have been mined in a small way. Feldspar and quartz are obtained 

 from pegmatites, of which there are many occurrences all through 

 the region, although seldom in workable masses. The largest quarries 

 are in southern Westchester cotinty. 



Taconic province. The Highlands on the east of the Hudson 

 river are succeeded northward by a series of ridges which cover 

 the whole area to the Massachusetts and Connecticut state line. 



