r>4 GREEN MOUNTAINS IN MASSACHUSETTS. 



Titanite, rutile, and tourmaline occur sparsely, as well as little broken prisms 

 of apatite and zircon prisms. The micas occur in homogeneous plates; the 

 interwoven sericitic structure is not common. Magnetite occurs occasionally. 



Another variety of these gneisses is distinguished by the evenness of 

 its character and its occurrence along the base of Hoosac mountain as the 

 most western band of the gneisses, in close connection with the quartzites 

 and limestones. The rocks thrown out from the "well" shaft, a few hun- 

 dred feet west of the west shaft of the tunnel, are typical of this variety. 

 In the hand specimen the rock is a fine grained, evenly banded grav gneiss; 

 the minerals are arranged in layers and the rock is filled with little 

 squarish feldspars. In the slide these feldspars are seen in gentlv rounded, 

 equidimensional crystals, in simple twins, according- to the albitelaw. The 

 groundmass is composed of little round or ellipsoidal quartz grains and 

 more angular pieces of feldspar (which are in part in simple grains, in part 

 doubly twinned microcline) mixed with threads of muscovite and biotite, 

 the whole so arranged as to produce a schistose structure in the rock. (See 

 PI. vu, a, and PI. vin, a.) Sometimes a band of mica and quartz cuts across a 

 feldspar, the two halves polarizing together and being therefore part of one 

 crystal The bands of the groundmass bend around the porphyritic feldspars 

 in gentle curves. These feldspars are honeycombed with little drops of 

 quartz and flakes of biotite and muscovite which are often arranged parallel to 

 the structure outside. Octahedra of magnetite are visible in the rock; 

 microscopic crystals of apatite, rutile, and zircon are abundant. In some 

 cases little grains of calcite occur abundantly, even included in the feld- 

 spars, and in some localities we find a calciferous gneiss with this same 

 structure, in which the groundmass contains a large amount of calcite in 

 little grains apparently homologous with quartz and feldspar. This variety 

 occurs at several places in the Hoosic valley near the junction between 

 the limestone and quartzite, and represents the Hoosac mountain gneisses 

 nearest to the limestone. 



At the base of the white gneiss series the rock in many places passes 

 so gradually into the underlying granitoid gneiss that it is impossible to 

 draw a line between the two. These varieties of the white gneiss are 

 very coarse and feldspathic, but the feldspars are white instead of red as in 



