266 PENTTI ESKOLA 



This gneiss has been regarded as of pre-Cambrian age and the 

 Cambrian and Ordovician rocks that bound it on the west and the 

 east have been thought to lie unconformably upon it. 



On its western side, in the Housatonic Valley, the main rock 

 is limestone, called Stockhridge limestone, interbedded with quartzite 

 and slate, while on the eastern side there is an area of highly 

 metamorphic schists. 



Within the area of the Becket gneiss there are several vertical 

 layers (up to six hundred feet broad) of limestone, which is older 

 than the gneiss. The most important of them, according to 

 Emerson, are situated at Coles Brook in Middlefield, north of 

 Becket Station in Becket, in Washington and Peru, in Hinsdale, 

 and in the valleys of East Lee, Tyringham, and Otis. All the 

 limestone formations within the Becket gneiss area are called 

 Coles Brook limestone from the type occurrence at Coles Brook in 

 the northeastern part of the mass. A striking topographical 

 feature of these occurrences of limestone is that they occupy long, 

 narrow valleys in the gneiss highland. 



Near the contacts with this limestone the Becket gneiss shows 

 two kinds of endomorphic contact phenomena: In many cases 

 the boundary type of gneiss grows more basic and is called Lee 

 quartz diorite. But in other places, the boundary type of gneiss 

 is rich in salic minerals and contains monoclinic pyroxene and 

 titanite. It is called by Emerson " titanite-diopside diorite aplite," 

 and is supposed by him to represent a product of assimilation of 

 the limestone by an igneous magma. 



During a trip to western Massachusetts in July, 1921, the 

 present writer, together with Dr. N. L. Bowen, had the opportunity 

 of studying outcrops and collecting samples of these contact rocks 

 around Benson Pond east of Washington Station, on the railroad 

 line between Washington and Becket stations, north of Becket 

 Station and in the valley of Goose Pond Brook near East Lee. 



As the problem concerning the assimilation of limestone by 

 igneous magmas is now being much discussed, this very instructive 

 case may be of some interest to petrologists. A brief note of field 

 observations and of a microscopic study of the materials collected 

 is therefore given in the present paper together with a discussion 

 of the results. 



