46 NEW YORK STATE MUSEUM 



ered as an alternative to the sedimentary theory of origin until more 

 definite knowledge on the subject is obtained. 



Some interesting trap dikes occur in the mine. They cut all the 

 rocks including the magnetite and have been described by Professor 

 Kemp as belonging to the camptonite class. A rather curious feat- 

 ure observed in some of these is the presence of calcite distributed 

 through the mass in small rhombohedra; this mineral occurs also 

 in some of the ore. The. largest dike is about 6 feet thick and 

 crosses the ore body diagonally. 



The ore from the Forest of Dean mine is medium grained, in- 

 clining to a shotlike texture, and unlike most of the magnetites in 

 this part of Orange county is quite free of admixture with silicate 

 minerals. It requires only rough sorting to prepare it for the fur- 

 nace; the waste is nearly all granite. The sorted ore carries 60 

 per cent or a little more, of iron. 



MICA 



The production of mica has never attained the basis of a settled 

 industry in this State, although small quantities are extracted from 

 time to time in an experimental way or as a secondary product in 

 the mining of other minerals. The occurrences illustrate the gen- 

 eral types of deposits which have commercial importance elsewhere 

 as sources of mica and are distributed over the two great crystalline 

 rock areas of the Adirondacks and the southeastern Highlands, be- 

 ing found in Orange, Putnam and Westchester counties in the High- 

 lands, and in Saratoga, Essex, St Lawrence, Jefferson, Herkimer 

 and other countries in the northern area. It is the purpose of the 

 present article to give some particulars regarding the character and 

 economic features of the deposits which hitherto have received lit- 

 tle more than casual attention. 



Mica is a general term for a group of minerals, of which three 

 varieties find commercial use. These include muscovite, biotite and 

 phlogopite. Muscovite is often called white mica, in allusion to its 

 transparent quality, but it is not necessarily light or highly trans- 

 parent, though the best commercial kinds are thus characterized. 

 Some muscovite from the Adirondack pegmatites has a smoky gray 

 color, nearly as dark as some examples of biotite. Chemically it 

 contains potassium and sodium as basic elements, and is therefore 

 an alkali variety. Biotite, as a rule, has a dark brown to nearly 

 black color, but occasionally is sufficiently light to be transparent 

 in thin sheets. Iron is present in considerable amounts and with 



