' THE MINING AND QUARRY INDUSTRY I908 85 



dip as the latter and are at times continuous for long distances, as 

 at Talcville where workable deposits extend fully a mile along the 

 outcrop. 



The derivation of the talc has been explained by C. IT. Smyth jr 

 as due to chemical alteration of silicates produced during the 

 metamorphism of the limestone. Originally an impure calcareous 

 sediment, the limestone under dynamic influences, has taken on a 

 crystalline character and became impregnated with silicates, chiefly 

 tremolite, actinolite and pyroxene. Certain beds seem to have con- 

 tained sufficient magnesia and silica to permit a complete trans- 

 formation into tremolite, forming a tremolite schist, while other 

 layers with a preponderance of lime have undergone a partial 

 change, showing scattered crystals and aggregates of silicates within 

 the crystalline limestone. Tremolite is always the principal silicate 

 in vicinity of the talc beds, though elsewhere pyroxene (enstatite 

 and diopside) is common. The change from tremolite to talc is 

 a result of weathering, and takes place readily under the influence 

 of ground waters holding carbon dioxid. A coordinate process has 

 produced serpentine from the same silicates, though this mineral 

 is seldom found with the talc, being confined so far as observed 

 to the outlying areas. The principle determining the nature of the 

 end product, whether talc or serpentine, seems to be a problem not 

 yet solved. The alteration of tremolite to talc, according to Van 

 Hise,^ may be given the following chemical formula : 



Ca Mg, Si, O,, -h H/:) + CO, = H., Mg, Si, O,, -f CaCO, 



The alteration is accompanied by an increase in volume of talc 

 and calcite amounting to 25.61 per cent, though if the talc alone is 

 considered there is a decrease of .83 per cent a> compared with the 

 tremolite. There is little or no calcite present in the talc ; it has 

 doubtless been removed by the water circulations during the process 

 of alteration. 



The beds under exploitation range from a few feet up to 50 feet 

 or more in thickness. They are inclined at varying angles, usually 

 between 30° and 60°, from the horizontal. The dip is uniformly 

 toward the northwest. Masses and seams of unaltered tremolite 

 are sometimes included within the deposits. 



Character of the talc. The characteristic product of the region 

 has a fibrous texture, belonging to the variety known as agalite. 

 It is in fact a pseudomorph of tremolite, to which the fibrous 



1 Treatise on Metamorphism, p. 286. 



