154 NEW YORK STATE MUSEUM 



Muscovite is found in Edwards associated with the fibrous talc. 

 It is not, however, of commercial importance. 



St Lawrence county may be regarded as one of the more favorable 

 sections for the occurrence of commercial grades of mica. Granite 

 intrusions of great size have taken place at different times in the 

 Precambrian and on their borders may be found dikes and lenses of 

 pegmatite, intersecting the older gneisses and schists. Since the 

 intrusion of the pegmatites there has been no great disturbance from 

 regional-metamorphic forces so that the mica is little fractured in 

 most occurrences, whereas the pegmatites in the central Adirondacks 

 often show the effects of severe compression. The pegmatites carry 

 both muscovite and biotite. The numerous contacts of limestone 

 and granite afford favorable conditions for the occurrence of 

 phlogopite, which, as stated, is found here and there in specimens 

 of commercial qualit}^, though the real importance of the deposits 

 has never been adequately tested. The geological relations in this 

 part of the Adirondacks are very similar to those in the mica-mining 

 districts of Canada. 



MILLSTONES 



The production of millstones or burstones is carried on in a 

 small way in the Shawangunk mountain area from the grit which 

 outcrops along the northern edge of the ridge. The industry has 

 been established a long time and for many years enjoyed prosperous 

 conditions through the extensive sale of the product for cereal mills. 

 This market has been gradually curtailed within the last quarter 

 of a century or more by the introduction of the roller mill process 

 for making flour, although some mills still retain burstones for 

 grinding the coarser cereals. At present the quarrying and shaping 

 of the stones gives employment to a small number of men who 

 engage in it during a few months of the year when not otherwise 

 employed. The product is sold chiefly among the small corn mills 

 in the south. Besides millstones, the quarries also turn out disks 

 of stone which are called chasers and are employed in the roll type 

 of crusher, the disks revolving on edge in a circular pan that is 

 sometimes paved with blocks of the same stone. This type of 

 crusher is used more or less in the pulverization of materials like 

 quartz, feldspar, barytes and mineral paints. 



Occurrence of grit. The Shawangunk grit is a hard firmly 

 cemented pebbly sandstone of light gray color. The beds range 

 from 50 to 200 feet, thinning progressively toward the northeast. 

 The. layers differ in texture, ranging from a medium-grained sand- 

 stone or quartzite to a coarse conglomerate in which the individual 



