Remnants of a chain of volcanic centers within the early Silurian-Early Devonian marine waters of the northern Appalachian geosyncline now are exposed along the New England coast and in southern New Brunswick. On the northwest the volcanic rocks lie unconformably on pelitic arenaceous and locally volcanic rocks of probably Cambrian-Ordovician age that generally decrease in metamorphic grade northeastward from Penobscot Bay, Maine, to the St. John River, New Brunswick. On the southeast in New Brunswick, the marine volcanic rocks are faulted against Precambrian metamorphic and Cambrian-Ordovician sedimentary rocks in the Kingston and Caledonian uplifts.

The coastal volcanic sequence is varied in thickness and lithologic succession. Maximum thickness is about 45,000 ft in the Eastport quadrangle of southeastern Maine. The volcanic rocks are largely fragmental--flow breccia, tuff breccia, and coarse bedded tuff. The rocks were originally of andesitic to rhyolitic composition but now are locally altered hydrothermally to keratophyre and quartz keratophyre. Basaltic flows, rarely pillowed, and coarse tuff are also present. Sedimentary rocks include shale and siltstone containing an abundant and varied shallow-water benthonic fauna, and poorly fossiliferous chert and fine-grained siliceous bedded tuff.

Folding, faulting, and plutonic intrusion of granitic and gabbroic magmas--the Acadian orogeny--occurred during Middle Devonian time and ended the volcanic activity. During Carboniferous and Triassic times, movements in a system of NE-trending faults, probably largely strike-slip, shuffled together blocks of different lithology, structure, and age that originally may have been many miles apart.

The lower Silurian-Lower Devonian volcanic belt thus marks the beginning of a zone of weakness in the continental crust beneath the northern Appalachian geosyncline. This zone persisted throughout the Acadian orogeny and continued into the Mesozoic Era, as expressed by a system of major faults.