48 



GREEN MOUNTAIN* IN MASSACHUSETTS. 



by metamorphism. On the other hand, its field relations show its close asso- 

 ciation with and frequent transition into coarse gneisses which seem to form 

 part of a detrital series. 



THE VERMONT FORMATION. 



A somewhat varied series of rock overlies this coarse basement gneiss. 

 At one place where there is no possibility of folding (namely, along the 

 pitching axis of Hoosac mountain (see PI. v, Profiles ix, x). The thickness 

 of this series has been measured between a conformable contact with the 

 granitoid gneiss below and one with the albite-schist above ; it is between 

 600 and 700 feet. 



Flo. 13.— Metamorphic conglomerate (Vermont formation). Dump, Central shaft. One-fifth natural size. 



This represents two faces of one block at right angles to each other, the line showing the corner. The pebbles are of 

 granulite ami blue quartz, sonic of them H inches in diameter. The different shape of the cross-sections in the two planes 

 is noticeable. By looking closely it will be Been that many pebbles are cut in two by dark lines (biotite), showing that 

 their present shape is due partly to crushing and the formation of new minerals. This is seen on the right side, but not on 

 the left. 



This formation contains an infinite scries of gradations between coarse 

 gneisses similar to the basement gneisses, finer grained banded gneisses, 

 gneisses composed of quartz and feldspar with but a small amount of the 

 micaceous element, metamorphic gneiss-conglomerates, ordinary quartzite- 

 conglomerates, and quartzites. This series of rocks (represented by gneisses 

 and metamorphic conglomerate) occupies a, position in the tunnel section 

 on either side of the central core of granitoid (Stamford) gneiss; while a 



