198 ECONOMIC GEOLOGY 



Potsdam sandstone of Cambrian age, deposited unconformably 

 upon the older formations. 



According to J. M. Clarke the metamorphic series consists of 

 the Grenville series of limestones and dolomites, gneisses of both 

 acidic and basic character bearing the common metamorphic 

 silicates together with graphite and pyrite; amphibolites com- 

 posed mostly of hornblende; quartzites and gneisses. The ig- 

 neous rocks consist of anorthosites, gabbros, syenites, granites, 

 with later diabase dikes. 



The normal magnetites occur both upon the eastern and the 

 western side of the Adirondack Mountains. One of the most 

 productive localities is Mineville, which is situated to the north- 

 west of Port Henry. The ores are granular masses of magnetite, 

 lenticular in character, but often in so nearly a horizontal 

 position, and of such length that they appear like bedded ores. 

 According to Kemp and Newland these magnetites are differ- 

 entiation products. Newland substantiates this theory by 

 noting, in the acid intrusives of the area involved, a large excess 

 of iron above the percentages required with the lime and 

 magnesia in the formation of silicates. 



The titaniferous magnetites of New York are found chiefly in 

 Essex and Franklin counties. The ore bodies occur in the intrusive 

 anorthosites which are traversed by dikes of gabbro. Small ore 

 bodies are found within the gabbro, in tabular form and con- 

 formable with the strike of the dikes. The larger ore bodies 

 in the anorthosites may represent differentiation products 

 formed during the solidification of the anorthosite magma, or 

 they may represent intrusives forced into the anorthosite after 

 a partial solidification of the anorthosite magma had taken place. 

 The ores bear both titaniferous magnetite and ilmenite, FeO, 

 TiO 2 . The gangue minerals associated with the ores are the am- 

 phiboles, the pyroxenes, olivine, and the metamorphic minerals 

 spinel and garnet. 



The Cambro-Ordovician belt finds its best representative at 

 Cornwall, Pennsylvania (see Fig. 104). The ore bodies of mag- 

 netite form large and small masses of somewhat irregular shape, 

 either within the sediments themselves, or at their contact with 

 intrusive Triassic diabase. While the'ores appear as contact de- 

 posits the characteristic silicates of contact metamorphic deposits 

 are largely wanting (Fig. 105). 



One of the best magnetite beds in northern Vermont is found 



