The Boundary Mountains terrane is defined primarily by a sialic basement consisting of a distinctive suite of diamictites, which were metamorphosed in late Precambrian time to granofels, gneiss, and schist. These rocks make up the Chain Lakes massif, exposed in the Boundary Mountains along the southwestern part of the Maine–Québec border, and large blocks of similar lithology exposed in mélange of the St. Daniel Formation, Eastern Townships of Québec. Rocks of similar lithology and age stand out as megaclasts in ophiolitic mélange near the northwest margin of the Macquereau dome, southeastern Gaspé Peninsula. The cratonal basement of the Boundary Mountains terrane may extend from central or northern New Hampshire and northeastern Vermont roughly 1,000 km to the western part of the Gulf of St. Lawrence, southeast of Gaspé. Collectively, these basement rocks are unlike those composing the Grenville tectonic province of the Laurentian Shield, and unlike high-grade gneisses exposed in the Miramichi Highlands of New Brunswick and in lithotectonic assemblages of Avalonian aspect bordering the Gulf of Maine and the Bay of Fundy. The accretionary history of the Boundary Mountains terrane is believed to have begun in Middle to Late Cambrian time. It therefore may represent one of the earliest of accretionary events in the prolonged orogenic history of the northern Appalachians. Two parallel mélange belts, the Hurricane Mountain and St. Daniel, of the Maine and Québec portions, respectively, of the northern Appalachians, are interpreted as suture zones that define the southeast and northwest margins of the Boundary Mountains Terrane. They are named for the predominant lithotectonic units in each belt—the Hurricane Mountain Formation, in the Lobster Mountain anticlinorium of Maine, and the St. Daniel Formation, which crops out along the southeast margin of the Baie Verte–Brompton line in Québec. The tectonic history of the Hurricane Mountain mélange belt is interpreted as expressing the amalgamation, during Late Cambrian to Early Ordovician time, of the Boundary Mountains terrane to a second terrane on its southeastern margin, probably the Gander. Sparse paleontologic and isotopic ages along the Hurricane Mountain belt indicate that suturing progressed from present-day southwest to northeast, along an ensimatic convergent plate boundary. Volcanogenic flysch deposits of the Dead River Formation, overlying the Hurricane Mountain Formation to the southeast, are believed to have formed in a forearc-basin environment. Polarity of subduction is inferred to have been toward present-day southeast. This diachronous event provides a tectonic driving mechanism, in time and space, for the Penobscottian orogeny. The Penobscottian event preceded the Taconian collision of the composite Boundary Mountains–Gander terrane to the Laurentian (North American) margin. Amalgamation of individual terranes, therefore, in this part of the northern Appalachians, did not proceed in a regular, craton-outward succession.

Three major pre-Silurian stratigraphic divisions are recognized in the rocks of west-central Maine: Division I, Precambrian crystalline rocks of the Chain Lakes massif; Division II, Cambrian(?) to Lower Ordovician(?) eugeosynclinal rocks; and Division III , Middle Ordovician to Upper Ordovician(?) eugeosynclinal rocks. The rocks are sparsely fossiliferous and are cut by four plutonic series, the oldest of which predates the Highlandcroft Plutonic Series. Division I consists of massive-bedded granofels and thin- to massive-bedded gneiss, metavolcanic rocks, schist, metasandstone, and amphibolite (~3,100 m). The granofels and gneiss are characterized by an unusual fragmental texture w h o se origin is obscure. Division II has four lithologic units that represent a transition from the sedimentary and volcanic components of an ophiolite sequence in the lower part to a flysch sequence in the upper part. The four units, in ascending stratigraphic order, are (1) greenstone displaying relict pillow structure, amphibolite, keratophyric metavolcanic rocks, and iron formation (~1,500 m); (2) quartz-rich metaquartzwacke and metagrawacke, magnesian metapelite, and greenstone displaying relict pillow structure (~1,100 m); (3) dark sulfidic interbedded metapelite, metagraywacke, feldspathic metasand-stone, and quartzite (~600 m); and (4) green and red magnesian metapelite, metagraywacke, and quartzite, all with characteristic pinstripe lamination (> 800m). Rocks in New Hampshire equivalent to those of Division II are intruded by the Highlandcroft Plutonic Series, which is similar to the Attean Quartz Monzonite, provisionally dated at about 470 m.y. The constancy of facing directions at contacts between units has made possible the definition of major fold axes and faults, and the definition of these structures has enabled us to make reasonable estimates of regional stratigraphic thicknesses. The stratigraphic succession established in this paper is believed to be reasonably comparable to ophiolite-flysch successions indicated from geophysical data and drilling in modern oceanic environments. If the comparison is valid, then the process of generating oceanic crust can be projected back to early Paleozoic time in this region. Units 3 and 4 of Division II have been traced directly to the Maine-New Hampshire border, where they are continuous with the Dixville and Albee Formations. This correlation involves either inversion of part of the section or overthrusting from either the southeast or northwest of the section where units 3, 2, and part of 1 would be sturcturally superposed on 4. Division III, which has been described elsewhere, probably conformably overlies unit 4 in the southwest part of the region.