Determination of the age of the metamorphosed sedimentary rocks in the southeastern portion of the Appalachian Mountains is difficult. Most of the fossiliferous Paleozoic strata of the northwestern, folded, Appalachians are separated from the zone of metamorphosed sediments either by fault blocks of Triassic sediments or by a barrier of pre-Cambrian gneisses. However, the individual lithologic units within the metamorphic zone maintain astonishing petrographic uniformity throughout long distances, so that correlations, possible at any one point, may have a bearing on the age of, even distant, large areas of metamorphic rocks.
In eastern Dutchess County, New York, about 70 miles north of New York city (Fig. 1), neither Triassic fault blocks nor pre-Cambrian gneisses present obstacles, and Cambro-Ordovician formations can be traced eastward, well beyond the westernmost zone of pre-Cambrian gneisses. This is looked upon as a key area, and, in surveying it, Balk has examined into . . .