ALGONKIAN ROCKS IN VERMONT. 427 



unless the conglomerate itself be taken as sufficient proof of an 

 unconformity. 



A practical difficulty was first met in finding a source for the 

 abundant pebbles of blue quartz which occur so plentifully in the 

 rock, and although sources for them are known, the proportion 

 of such material seems to bear no proper relation to the known 

 extent of rocks in the Mount Holly series that would be likely 

 to yield pebbles of this mineral. Reference has already been 

 made to a coarse phase of the conglomerate near South Chitten- 

 don where its clastic quartz best deserves the name of boulders. 

 Such coarse phases are exceptional. An unusually coarse variety 

 occurs one mile north of Mendon village. With the quartz peb- 

 bles there is a plentiful sprinkling of gneiss pebbles, varying in 

 size from small grains up to two feet in diameter. Clastic areas 

 of orthoclase are also numerous ; pebbles two inches in diameter 

 being the largest. Under the microscope abundant small grains 

 of detrital feldspar can be detected. At this locality the 

 original character of the rocks seems best preserved of anywhere 

 that it is known to me, and a careful comparison of its gneissic 

 elastics with the gneisses of the lower series immediately subjacent 

 was made in hopes of being able to refer the pebbles to their 

 sources. Macroscopically there appears to be no doubt that most 

 of the pebbles were derived from the complex of gneisses to the 

 east, and in the days before microscopical methods were used such 

 a source would have been unhesitatingly affirmed. But today the 

 microscope instead of simplifying one's difficulties apparently only 

 adds to them. It is seen that the conglomerate here has recorded 

 the evidence of dynamic action to a somewhat less extent than 

 in many localities, but still an evident effect of metamorphism is 

 observed. The micro-study of the lower gneiss shows them to be 

 coarse to fine, irregularly-laminated orthoclase rocks in which both 

 quartz and feldspar are badly crushed and distorted. About the 

 resulting mosaics have been developed abundant epidote and 

 titanite crystals and patches of biotite, colorless muscovite and 

 chlorite. In the clastic gneiss little or no epidote or titanite can 

 be detected, while there is always present more or less pale-green 



