84 . NEW YORK STATE MUSEUM 



Occurrence 



Deposits of iron ores are abundantly distributed, and it is im- 

 practicable here to give more than a few of the principal facts 

 relating to the occurrence. They are, naturally, grouped into more 

 or less definite areas or regions determined by geologic and geo- 

 graphic boundaries. Nearly all the workable deposits occur in the 

 following areas. 



1 Adirondack region. Magnetite ores with occasional hematite 

 (martite), in Precambric gneisses. Also titaniferous magnetite in 

 basic igneous rocks of the gabbro family that are intrusive in the 

 gneisses. 



2 Highlands of the Hudson. Magnetite in Precambric gneisses. 



3 St Lawrence and Jefferson counties. Hematite associated 

 with crystalline limestone, serpentine and schist of Precambric 

 (Grenville) age. 



4 Central and western New York, Fossil and oolitic hematite 

 interbedded with limestones and shales of the Clinton group. 



5 Dutchess and Columbia counties. Limonite associated with 

 crystalline limestones, slates and schists, representing metamor- 

 phosed Cambro-Siluric strata. Siderite, the carbonate of iron, some- 

 times accompanies the limonite and in one group of mines it is the 

 principal ore. 



6 Staten Island. Bog ore (limonite) occurring in superficial 

 deposits resting on serpentine. 



I The nontitaniferous magnetities of the Adirondacks are found 

 on the outer borders in Washington, Warren, Essex, Clinton, 

 Franklin and St Lawrence counties. On the southern side they 

 are less prominent ; the Salisbury mines in Herkimer county, how- 

 ever, belong to this type. The country rock is chiefly gneiss, with 

 bands of crystalline limestone, quartzite and schists that are 

 undoubted sediments. The gneiss shows great variation from place 

 to place, but in the vicinity of the ore bodies it is prevailingly a 

 rather acid rock of ligh color, composed of alkali feldspar, quartz 

 and one or more dark silicates, most commonly augite and horn- 

 blende. Its mineralogy is that of granite or syenite. Though the 

 great mass is of uncertain origin, some phases of the gneiss have an 

 igneous character. A quartz-plagioclase gneiss and basic horn- 

 blende gneisses are found over limited areas. The ore bodies lie 

 along or near the contact of two varieties of gneiss, or of gneiss 

 and gabbro, and again are apparently in the interior of a gneiss 

 belt. They conform closely in dip and strike to the lamination 



