HOOSAC MOUNTAIN. 



51 



of these crumbly quartzites show but little feldspar and that quite fresh, 

 while the quartz grains show in the slide abundant signs of great pressure, 

 or even crushing. Some of these quarries are located at sharp folds of the 

 quartzite, so that the crumbly nature of the quartzite may be in part due 

 'to a mechanical loosening of the cohesion. 



The pure conglomerate-quartzites occur often in the quartzite; for 

 instance, the quartzite resting on the granitoid gneiss (Stamford gneiss) of 

 Clarksburg mountain, in which Mr. Walcott has found fragments of trilo- 

 liites. 1 contains pebbles of blue quartz, which are often only distinguishable 



SbS 5 '? 



Fig. 16 Metamorphic conglomerate (Vermont formation). Dump, Central shaft. One-sixth natural size. 



The pehbles are mostly grarmlitic, but there are some of blue, and some of white quartz. In this type we have round 

 and flat pebbles occurring together, the round ones differing but little in shape in the normal plane. This* represents 

 the typical variety of conglomerate. 



by their color from the surrounding quartzite cement. The microscope 

 shows that many" of these pebbles are composed of an aggregate of little 

 quartz grains derived from a homogeneous mass by crushing, and hence 

 they easily blend with the quartz cement of the rock. They occur often 

 in flattened elongated forms which it is difficult to distinguish from secre- 

 tions. (See PI. x, b.) 



Some of the quartzites contain abundant calcite grains arranged in 

 stringers, and scattering flakes of muscovite. 



Those quartzites in which feldspar becomes more prominent preserve 



1 Am. Jour. Sci.,3d ser., vol. 35, 1888, p. 236. 



