404 THE JOURNAL OF GEOLOGY. 



of his father, as to its anticlinal structure, and cites as proof the 

 supposed equivalence of the " Potsdam " and "Levis" rocks on 

 both sides of the range in Wallingford and Plymouth/ 



THE PROBLEM OUTLINED. 



From the opinions held as to the age, character, and structure 

 of the Green Mountain axis just given, the main facts that stand 

 out most prominently are that the centre of the mountains is 

 occupied by strata to which the name gneiss is universally given, 

 and that bordering this, on the west, occurs a terrane variously 

 called "granular quartz," "quartz rock," and "quartzite," by differ- 

 ent authors, together with an associated conglomerate. These last 

 two rocks have been referred to various horizons from the Azoic 

 to the Medina sandstone. Most geologists have grouped the 

 central gneiss among the oldest, although Thompson considered 

 it more recent than the Stockbridge limestone. 



The relations of the conglomerate to the quartzite are by no 

 means so simple as the older geologists were disposed to believe. 

 Between the conglomerate and the quartzite there is an extensive 

 series of metamorphosed sedimentary rocks which have been over- 

 looked in the past, and which are in part the subject of this paper. 

 Beneath the conglomerate horizon the gneisses and other rocks 

 occurring in the amphitheatres, with their interstratified limestones 

 and quartzites make a second series composed wholly or partly of 

 sedimentary rocks separated from the first, of which the con- 

 glomerate is the base, by an unconformity sufficiently well iden- 

 tified to warrant a sub-division of the Pre-Cambrian Algonkian 

 terranes into two series. 



REASONS FOR REFERRING THESE ROCKS TO THE ALGONKIAN. 



It is due to the labor of Mr. Walcott that the age of the 

 quartzite on the western border of the range has finally been 

 determined definitely. Upon palaeontological evidence he refers 

 it to the Lower Cambrian horizon and makes it equivalent to the 

 red sand rock of Georgia, Vermont ; the latter being an off- 

 shore, and the former a near-shore deposit. In his Cambrian 



' The Geology of Vermont, Proc. Amer. Assoc. i6th meeting, 1886, p. 121. 



