Il6 NEW YORK STATE MUSEUM 



2 Highlands of the Hudson 



The belt of Precambrian gneisses which crosses southeastern New 

 York from New Jersey to the Connecticut boundary, occupjdng a 

 large part of Orange, Putnam and northern Westchester counties, 

 contains many magnetite bodies that have been mined on a small 

 scale, altogether constituting a resource of considerable importance. 

 The deposits are strung out along a northeast-southwest axes parallel 

 with the general structural trend. Geologically they exhibit a wide 

 range of features and are also varied in their mineral and metal- 

 lurgical characters. 



The gneiss belt is composite, with some types certainly of igneous 

 derivation and others that are very likely sedimentary. The igneous 

 members include granitic, syenitic and dioritic gneisses, accompanied 

 usually by more massive pegmatitic phases. Throughout the 

 Highlands there is extensive development of pegmatitic granite in 

 bands, schlieren and small bosses and it is almost invariably encoun- 

 tered in the vicinity of the magnetites. Of the sedimentary gneisses 

 there are very few areas where they are exposed in unmixed develop- 

 ment, for nearly everywhere they are interleaved and injected by 

 magmatic material of similar nature to the larger bodies of igneous 

 gneisses. Their usual habit is a banded one, with alternating layers 

 of lighter and darker color corresponding to variations in mineral 

 composition. Some layers are nearly pure quartz, and others are 

 largely hornblende or mica, or a mixture of these with feldspar and 

 quartz. These sedimentary'- gneisses may include more than one 

 stratigraphic group, as their areal relations have not been fully 

 studied, but in many places they may be said to show close resem- 

 blance to the Fordham gneiss that is extensively developed in 

 southern Westchester county and that has been classed by Berkey 

 as early Precambrian and likely equivalent to the Grenville gneiss 

 of the Adirondacks. Small bodies of crystalline limestone accom- 

 pany the gneiss and are to be regarded as a part of the same sedi- 

 mentary series. The igneous intrusions in the Highlands belong 

 mainly to the early Precambrian and have undergone compression 

 and extensive structural rearrangement. The Cortlandt series of 

 basic intrusives lies on the southern margin of the Highlands proper 

 and has not shared in the general metamorphism, being of 

 Paleozoic age. 



The magnetites, hitherto mined, have been shipping ores with 

 from 50 to 60 per cent iron. These are not yet exhausted, although 

 the main supply for future mining consists of concentrating ores 

 which carry upward of 25 per cent iron and which can be worked 

 on an extensive scale. The richer magnetites, with few exceptions 



