350 C. L. Whittle — Main Axis of the Green Mountains. 



The araphibolites are cut by dikes of the same rock and also 

 by more modern dikes of camptonite that are younger than 

 the shearing. Such a series of amphibolites probably repre- 

 sents a period of volcanic activity vastly older than the Cam- 

 brian and of greater areal distribution than occurs at present. 

 Further south, reconnoissance work has not detected them ; 

 they may have been eroded, while to the north, if they exist 

 they are covered, except in two isolated localities by the sedi- 

 mentary series lying immediately below the lower-Cambrian 

 quartzite. Mr. Wolff has described an amphibolite from a 

 hill situated about one mile south of Mt. H0II3 7 station and it is 

 referred with probability to an original diabase by him in his 

 forthcoming monograph on the Geology of Hoosac Mountain.* 

 As mentioned above, dikes of the same rock traverse the 

 amphibolites and possess the same local schistosity. There are 

 many other areas of this rock in the region some of which 

 are undoubtedly dikes, others owing to their extent are con- 

 sidered to be intrusives or surface flows. Their abundance 

 may be cited as evidence of surface flows since it is improbable 

 that any area, reasoning from analogy, would be traversed by 

 so large a number of intrusives. This view is also sustained 

 by the fact that diabases are prevailingly surface flows, such 

 regions as Connecticut, New Jersey, and Keweenaw Point be- 

 ing examples. It may also be mentioned in this connection 

 that in the border series of elastics I have observed no amphi- 

 bolites. Diabases of any age are rare in Yermont and one 

 would expect that were the amphibolites originally intrusive 

 (assuming that the border series is not separated from the core 

 by an unconformability) that similar areas would occur in rock 

 stratigraphically higher than those under consideration. Below 

 the associated rocks are referred to the Algonkian. As the 

 Keweenawan with its abundance of surface volcanic rocks was 

 presumably laid down near the close of the Algonkian time it 

 is not unlikely that similar associated flows took place in the 

 Algonkian rocks of Yermont. 



Following the classification of pre-Cambrian rocks now 

 adopted in this country none of this area studied in Yermont 

 can be referred to the Archaean and must be placed in the 

 Algonkian. The lowest rocks exposed in Shrewsbury and Mt. 

 Holly, although of extreme age and gnarled and crinkled into 

 a hopelessly involved structure still in larger part reveal their 

 sedimentary origin by associated beds of crystalline limestone, 

 now altered in part to serpentine or amphibole, and scattered 

 outcrops of quartzite. With these two rocks or their representa- 

 tives are biotite, augen gneisses and schists, garnetiferous mica 



* Part 3, Geology of the Green Mountains in Massachusetts by R. Pumpelly, 

 J. E. Wolff, T. Nelson Dale and Bayard T. Putnam, submitted in 1889. 



