GNEISS AND LIMESTONE CONTACT PHENOMENA 293 



the country-rock. This is in accordance with the experience that 

 assimilation usually occurs where the intrusive rock shows banded 

 structure and a high degree of protoclastic crushing and shearing, 

 while it does not occur in masses which have been intruded and 

 crystallized under quieter conditions and have come in contact 

 with the country-rock along smooth surfaces only. 



It does not seem, however, that the whole question can be 

 covered by this explanation only. There are no doubt differences 

 in the physicochemical conditions, in the concentration of volatile 

 compounds, etc., which cause assimilation to occur in one case and 

 not in another. Such relations may be understood better as soon as 

 more experimental evidence has been gathered about the solubility 

 of carbonates in magmas. 



SUMMARY 



Within the area of the igneous Becket granite gneiss in western 

 Massachusetts there occur several tilted up layers of crystalline 

 limestone, called Coles Brook limestone, older than the gneiss and 

 metamorphosed by its contact influence. In the vicinity of the 

 limestone the gneiss contains considerable quantities of lime- 

 bearing silicates, especially of clinopyroxene (diopside-hedenbergite) 

 and titanite, apparently the result of assimilation of limestone by 

 the gneiss magma. 



The gneiss is markedly banded, with alternating darker and 

 lighter bands. It was found, by determining the refractive indices 

 of the chief mafic minerals, biotite, clino-amphibole, and clino- 

 pyroxene, that the amount of their magnesia compounds in propor- 

 tion to their ferrous compounds increases with the total quantity 

 of the mafic constituents. At the same time the amount of anor- 

 thite in the plagioclase increases. Thus the dark bands behave 

 like the earliest separated rocks in a differentiation series. Some 

 differentiation by crystallization really seems to have taken place 

 after the assimilation. In certain places, however, and especially 

 at the immediate contacts against the limestone, the actual compo- 

 sition of the gneiss appears to be a direct result of assimilation and 

 no correspondence between Fe:Mg ratio and "basicity" exists.^ 

 The distribution of magnesia and ferrous oxide among the different 



