Page:The Journal of geology (IA journalofgeology21894univers).pdf/420

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.^[1]

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 different 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 overlooked 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 conglomerate is the base, by an unconformity sufficiently well identified to warrant a sub-division of the Pre-Cambrian Algonkian terranes into two series.

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