270 PENTTI ESKOLA 



LIMESTONE AND SKARN^ 



Emerson characterizes the Coles Brook limestone in the follow- 

 ing words f ''The Coles Brook is a coarse, highly crystalline, 

 magnesian limestone, locally white and pure, generally graphitic 

 and greatly changed to a mass of silicates — chondrodite, wollas- 

 tonite,^ wernerite, hypersthene, pyroxene, amphibole, titanite, 

 adularia, pericline, and others " 



In the following are given the writer's observations on the 

 limestones in western Massachusetts. 



About ^ mile east of Benson Pond, i mile east of Washington 

 Station, a band of limestone, a few meters broad, occurs in the 

 gneiss. It is rendered impure by the presence of greenish white 

 mica, reddish brown chondrodite, and clear brown titanite. 



The mica is nearly, though not quite, uniaxial and has 180 = 70 = 

 1.576=1=0.002. It is a magnesian mica, very poor in iron, but 

 probably not phlogopite. 



The chondrodite is notably pleochroic, a. and /3 reddish brown, 

 7 pale brown, a A jS = 3i°'a;i5 = 1-621=^0.003; /3d = 1.632=1=0.001; 

 70 = 1.655=^0.003. 



This limestone is a calcite rock and contains no dolomite. 



In apparent connection with the limestone, on its continuation 

 along the strike, were found many inclusions of clinopyroxene 

 skarn in an aplitic variety of the gneiss which contains varying 

 amounts of the same kind of pyroxene (Fig. i) as the skarn. 



Many of the inclusions are sharply defined, coarsely crystalline 

 masses of grayish green clinopyroxene, although narrow veinlets 

 always protrude into them from the aplite. These veinlets always 

 contain minute grains of red grossularite-andradite which is also con- 

 centrated within a narrow zone around the inclusions. It is appar- 



^" Skarn. — An old Swedish mining term for the silicate gangue (amphibole, 

 pyroxene, garnet, etc.) of certain iron ore and sulphide deposits of Archaean age, 

 particularly those which have replaced limestone and dolomite. The term is used 

 in this sense by Fennoscandian geologists, but it has been extended to cover analogous 

 products of contact metamorphism in younger formations." A. Holmes, The Nomen- 

 clature of Petrology (London, 1920), p. 211. 



^ B. K. Emerson, U.S. Geol. Surv., Bull, ^gy, p. 21. 



3 This statement about wollastonite does not occur in the other more detailed 

 reports and may be erroneous. 



