Anticlinorial massifs, composed of rocks which in part give radiometric dates of approximately Grenville age, are developed in western New England and southeastern New York at the boundary of the eugeosynclinal sequence on the east with the miogeosynclinal sequence on the west. The latter is overlain by rocks of the Taconic allochthon, consisting of eugeosynclinal deposits thrust from east to west.

A multiple N-S-trendlng linear series of domes, east and west of the Connecticut Valley, lies between the Merrimack synclinorium and the western massifs. The eastern part of the area is composed of a series of imbricate fault blocks bounded by west-dipping thrusts. An eastern series of basins, consisting chiefly of Paleozoic rocks of relatively low metamorphic grade, is developed on a Precambrian basement. This basement complex, giving ages of approximately 600 m.y., is younger than that of the western part of the map area.

An analysis of structural data from the Wachusett-Marlborough fault block indicates that mapping of joints may provide an indirect method for extrapolating the presence and attitude of covered faults of regional significance. The writer theorizes that the zone of batholiths of the Merrimack synclinorium and the fringing zones of domes composed a Paleozoic rise. Upward and outward expansion of the rise produced syn- to postorogenic gravity slides and thrust faults. Collapse of the rise in early Mesozoic time produced adjustments in the attitudes of the faults such that the east- and west-dipping faults assumed, respectively, low-angle westerly dips in the eastern part of the area and easterly dips in the western part, end now are properly classified on the basis of both their geometry nd movements as reverse or thrust faults.