PETROGRAFHICAL DESCRIPTION. 29 



THE LEE GNEISS. 



Amphiholitv fnnu Wasliiiio-tou.' Black, fine-orained, distinctly luMldcd 

 rock. Microscopic li(>nil)lende abundant in small, thin plates, of nicdiiini 

 absorption and pleoclu-oism. c=b>a; c=pale indi<>-o; b=olive; a=pal(' 

 oclier. Menaccanite in large, shapeless masses, with broad Ijordcr of 

 leucoxeue, abundant; little biotite. 



The common feldspathic mosaic forming the groundmass of the rock 

 is so covered up by the hornblende blades and of so fine grain that it is 

 not possible to determine the variety of plagioclase which is present. There 

 is not the slightest trace of cleavage or twanniug, and thus there is small 

 ground to suppose the rock to have been greatly influenced by shearing forces. 

 At the same time, the separate rounded or polygonal grains of which the 

 mosaic is composed show quite uniformly, when examined witl' j)lane- 

 polarized light, a form of undulatory polai-ization which I have called in 

 the following notes concentric polarization. A single grain becomes black, 

 first at the border, and the darkening advances regularly toward the center, 

 and it sometimes requires a rotation of 45° to render the whole fragment 

 dark. At times such a fragment is cracked into several parts without 

 disturbing the regularity of the above process. 



In the absence of cleavage and twinning it is not possible to think of 

 this as a resvilt of strain from the external forces which have deformed the 

 rock. It also is without the banded zonal arrangement which usually 

 acconqjanies changes of chemical composition, and where a distinct crystal 

 has Ijeen broken up into such a mosaic the fragments show this peculiarity 

 in a striking manner. It is a structure characteristic of the whole series of 

 amphibolites descril)ed in the following pages, and especially of several forms 

 which are certainly derived from limestones. This amphibolite preserves no 

 residual structures })ointing to an eruptive origin. It is a long, interbedded 

 stratum, parallel with and near to the Hinsdale limestone, and it is a distinct 

 associate of this rock and reappears with it in the Coles Brook band. It 

 occurs also as a continuation of the limestone seen on the Alderman farm in 

 Becket, where in one place the limestone is changed into white tremolite- 

 schist for 7 feet in from the contact and in another into black amphibolite. 

 It is also seen at the interesting outcrop in i\Ii<ldlefield described above; 



' C. F. I.ymau'.s pasture, east of the graphite mine. 



