GNEISS AND LIMESTONE CONTACT PHENOMENA 287 



of dolomite, calcite, and tremolite, with some brown mica 

 (/3d = Td = 1. 582 =i= 0.003), but no quartz. It thus proves to represent 

 a true equihbrium under those temperature-pressure conditions 

 which give rise to tremoHte limestone. 



Diopside limestone occurs so commonly that it is not necessary 

 to name more than a few localities, as examples. Emerson^ 

 describes pyroxene- and actinolite-bearing limestone from Hinsdale 

 and East Lee, both being Coles Brook limestone. His "Mineral 

 Lexicon of Eastern Berkshire Co." (op. cit.) mentions pyroxene 

 from the Stockbridge limestone from New Marlboro and several 

 places in Tyringham. 



The writer studied diopside limestone from Becket Station, 

 a combination of calcite, quartz and diopside. It is a rock of the 

 diopside limestone type. All the dolomite has been exhausted in 

 the formation of diopside, while some quartz has been left. 



Theoretical reasoning leads to the conclusion that the quartz- 

 carbonate rocks, when they are metamorphosed under medium 

 temperature conditions, corresponding to the tremolite and diopside 

 limestone types, are likely to become poorer in dolomite. This 

 must finally cause a dedolomitization of the limestone. 



The silica need not occur within the limestones themselves, 

 but may react at the boundaries. In small bodies of limestone 

 bounded by highly siliceous rocks, and especially when submerged 

 in magmas of such acid rocks, this may be expected to give rise to 

 a perfect dedolomitization. So far as the writer's experience goes, 

 small bodies of limestone inclosed in granites or gneisses really 

 consist of calcite-rock, while magnesium-bearing silicates occur 

 at their contacts. This rule was confirmed in the Massachusetts 

 localities studied, near Benson Pond and near Becket Station. 



ASSIMILATION OF LIMESTONE BY GRANITIC AND PEGMATITIC MAGMAS 



The writer believes that the mode of occurrence of the clino- 

 pyroxene gneiss around and near the occurrences of limestone 

 included in the gneiss is in itself sufficient evidence that the gneiss 

 has received its excessive amount of lime through assimilation of 

 the limestone. 



' U.S. Geol. Siirv., Bull, i^g, pp. 32 and 51. 



