Page:The Journal of geology (IA journalofgeology21894univers).pdf/443

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 Chittendon 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 pebbles 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 clastics 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