SOME PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 187 



ment of ores, probably resulted from one brief period of 

 deformation and intrusion. 



Koeberlein' describes the Brewster district as a part of the pre- 

 Cambrian area of New York City, about 54 miles to the northwest 

 of the city. The abandoned Tilly Foster, Brewster, and Croton mines 

 are located here. The stratigraphy from the bottom is reported as 

 Fordham gneiss comprising three series, sedimentary, granitic, and 

 syenitic, respectively, followed by the Inwood hmestone and the 

 Manhattan schists intruded by diorites and pegmatite. 



The ores are titaniferous and non-titaniferous magnetites, closely 

 related to the syenite of the Fordham gneiss. The syenite contains 

 magnetite, apparently the last mineral to crystallize. The titani- 

 ferous magnetites are regarded as magmatic segregations. The non- 

 titaniferous ore of the Tilly Foster mine is regarded as a replacement 

 of limestone by solutions given off during the cooling of the syenite. 

 The gangue consists of chondrodite, garnets, and other minerals 

 characteristic of the deep-seated metamorphism of limestones. 



W. J. Miller^ states that the oldest rocks of the Remsen and 

 Port Leyden Quadrangles of the southwestern part of the Adiron- 

 dack Mountains are the Grenville sediments, originally sandstones, 

 shales, and limestones which were altered to gneisses and crystalline 

 limestones by syenite intrusions and complex folding, before 

 Cambrian time. 



Stose^ states that the pre-Cambrian rocks of the Mercersburg- 

 Chambersburg area of Pennsylvania consist of a series of altered 

 basalt flows overlaid by altered, finely laminated, spherulitic, and 

 porphyritic rhyolite lava. 



Watson"* reports that an insignificant tonnage of manganese ore 

 has been mined in the crystalline schists of Georgia, which are at 



' F. R. Koeberlein, "Brewster Iron-bearing District of New York," Econ. GeoL, 



IV (1909), 714-54- 



^ W. J. Miller, "Geology of the Remsen Quadrangle," New York State Museum 

 Bull. 126, 1909, 51 pp., II pis., 4 figs., I geol. map; "Geology of the Port Leyden 

 Quadrangle," New York State Museum Bull. 135, 1910, 61 pp., 5 figs. 



3 G. W. Stose, Mercersbiirg-Chambersburg Folio, U.S.G.S. Folio 170, 19 pp., 8 pis., 

 5 figs., 1909. 



1 Thomas L. Watson, "The Manganese Ore Deposits of Georgia," Econ. Geol., IV 

 (1909), 46-55- 



