56 GREEN MOUNTAINS IN MASSACHUSETTS. 



The rock contains pebbles of two varieties: one kind composed of bright 

 blue opalescent quartz, the other resembling a fine grained granite, composed 

 of quartz and feldspar in small grains, speckled with biotite. These pebbles 

 on the average arc as large as a walnut, though some are much larger, 

 and they diminish in size until undistinguishable from the elements of the 

 groundmass. The shape is sometimes round, sometimes ellipsoidal, angular, 

 or flattened. In Fig-. 13, which gives two sides of a large block, the different 

 cross-section of the pebbles in two planes is shown. The groundmass or 

 cement outside is composed of smaller grains of blue quartz, small feldspars 

 resembling the albite of the schist, and biotite and muscovite in large amount. 

 The effects of crushing in the rock are evident; the pebbles are often trav- 

 ersed by parallel breaks or oblique cracks by which bands of biotite pene- 

 trate them, isolating parts of the pebble. Sometimes a pebble is cut in two 

 across its axis by such a band of mica. Thus pebbles, in appearance sepa- 

 rate, may have been parts of one individual originally. This crushing action, 

 combined with the formation of the biotite bands, gives many of these origi- 

 nal pebbles flattened shapes, so that they appear as layers of granitic mate- 

 rial cut off by the biotite bands in planes oblique to their trend. Some 

 varieties of this conglomerate gneiss have a banded structure, due in large 

 part to this crushing action carried to an extreme. (See Fig. 17.) In some 

 cases the pebbles are single crystals of feldspar, and this is occasionally 

 microcline. 



Figs. 13-20 show the character of this rock. Some of the pebbles 

 consist of fine grained granite containing small grains of blue quartz. Fine 

 grained gneissoid layers corresponding to the cement often alternate with 

 pebbly layers (see Fig. 18). In some varieties these granite pebbles lie in 

 a very micaceous matrix, composed of small feldspars resembling those of 

 the schist; in others the pebbles become so small that we get an even banded 

 gneiss containing larger grains of blue quartz, the whole forming- the ordi- 

 nary white gneiss previously described. It is then very difficult or impossible 

 to separate the old quartz and feldspar from that formed in situ. The opal- 

 escent blue quartz pebbles always retain their round form and are rarely 

 entered by the biotite outside. This shows perhaps a connection between the 

 f irmation of the biotite and the feldspar substance. The previous description 

 is based on the conglomerate of the tunnel dumps. 



