THE MINING AND QUARRY INDUSTRY IQII 9^ 



from Other minerals. The tremoHte is white or light gray in color 

 and is usually developed in finely fibrous individuals which when 

 felted form a compact and tough rock. The pink variety known as 

 hexagonite is of limited occurrence. Bands and irregular masses of 

 the tremolite occur within the talc deposits, and the immediate walls 

 generally consist of the schist, the border being marked by alter- 

 nating layers of talc and schist. 



The association is suggestive of the derivation of the talc which 

 has been the subject of study by C. H. Smyth, jr.^ The tremolite 

 is no doubt the parent mineral. As explained by Professor Smyth, 

 the limestones were originally impure calcareous sediments and by 

 metamorphic influences have taken on a crystalline character and 

 became impregnated with siHcates. Certain limestone beds seem 

 to have contained sufficient magnesia and silica to permit their 

 complete transformation to 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 limestone. The subsequent change of tremolite to talc 

 is the result of weathering and takes place through the agency of 

 ground waters holding carbon dioxid. The alteration may be formu- 

 lated chemically as follows : CaMgsSi^ Oi. + H.O -f COo = Ho 

 MggSi^ Oi2 + CaCOg. The change 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 as com- 

 pared with the tremolite. There is little or no calcite in the talc, 

 so that it probably has been removed with the progress of the 

 alteration. 



The talc is really a pseudomorph after the tremolite and it is due 

 to this that it possesses a fibrous character. Microscopic examina- 

 tion of specimens from almost any of the mines will show a little 

 residual tremolite in the centers of the fiber aggregates, and in some 

 samples there is a very considerable proportion of unaltered mineral. 

 Foliated talc accompanies the fibrous variety, being more abundant 

 apparently the farther the process of alteration has gone. It is of 

 course a separate development deposited by the circulating waters 

 which have taken the materials of the schist into solution. 



The view that the tremolite has been formed by metamorphisni 

 from the ingredients of the limestones without addition of material 



1 Report on four townships in St Lawrence and Jefferson counties, N. Y. Stale 

 Mus. Rep't 47, 1894, p. 491-515. Also, Report on the Tale Industry of St 

 Lawrence County, N. Y. State^Mus. Rep't 49-2, 1898, p. 661-71. 



