SOOSAC MOUNTAIN. 57 



On the surface here and there conglomerates are found, often associ- 

 ated with quartzites; in the latter case the pebbles are all quartz and the 

 cement is composed of biotite, muscovite, small feldspar, and magnetite 

 crystals. 



( hi the crest of Hoosac mountain, in Profile i\. PI. v. the conglomerate 

 is represented principally by the finer grained varieties, but toward the base 

 the pebbles are much larger and are in part not pebbles, but fragments of 

 layers broken up by crushing (see Figs. 15 and 27), giving angular forms. 



When we pass westward from the crest of Hoosac mountain, where 

 the conglomerate lies in its normal position, we trace the rock into the 

 white gneiss series on the slopes of the mountain. The pebbles have lost 



Fig. 19.— Metamorphic conglomerate (Vermont formation). Dump, Central shaft. About one-seventh natural size. 

 In ihis variety the pebbles are of much larger size {over 5 inches long), they have the mosi perfei I beach-pebhle .shape, 

 and axe composed of a very fine grained granite, which contrasts sharply with th<- much coarser gneissoid cement i omposed 



of quart/ ami feldspar grains ami mica flakes. The long, white, irregular masses in the center are secondary vein quartz 



their distinctness, and without the favorable exposures on the summit and 

 from the tunnel we would not suspect their nature; thev appear as white, 

 flat, lenticular masses of quartz and feldspar, which only in rare places sug- 

 gest a conglomerate (see PI. x, b), but when one has traced this rock foot 

 by foot into the conglomerate he recognizes the pebbly look at once. It is 

 apparent that this change is connected with stretching of the rock, for the 

 conglomerate is folded over and then turned under on the west flank of the 

 mountain. 



The microscope shows that the quartz pebbles are homogeneous masses 



