The authors are indebted to C. W. Harper, Jr:, for the use of material from his doctor's thesis on the brachiopods of the Arisaig Group. In that study, he identified many of the small collections that have helped to make correlation possible between the shore and the inland areas. W.B.N. Berry and M. J. Copeland have greatly assisted by identifying the graptolites and the ostracods, respectively, found at Arisaig. R. K. Bambach has generously contributed his knowledge of the Arisaig bivalves; he has also permitted examination of Twenhofel's field notebook. The chapter on the Knoydart has greatly benefited from the work of R. H. Denison, D. J. MacNeil, A. Suparman, and M. Tauchid. John F. Dewey gratefully acknowledges financial support from the University of Manchester, the Royal Society of London, and the Nova Scotia Research Foundation. D. Kelley has been generous with information on the Cobequid Mountains. We are indebted for funds received by W. S. McKerrow from the Nova Scotia Research Foundation, the Royal Society of London, and the Geological Society of London Tyrrell Fund, and by D. L. Dineley from the Geological Survey of Canada and the National Museum of Canada. McKerrow spent the year 1964–1965 as a post-doctoral Research Fellow at the California Institute of Technology with support from the National Science Foundation (Grants GP-2290 and GP-3743). Acknowledgment is made in the list of maps to those Massachusetts Institute of Technology students who contributed to the brook maps presented in this report, but thanks are due

The richly fossiliferous Silurian and Early Devonian rocks of the Arisaig area and the nearby related regions of Cape George and Lochaber in Antigonish County, Nova Scotia, have been known for almost one hundred and fifty years. The 5,000 ft of sediment in the Arisaig Group is the only fossiliferous section within the Province containing representatives of every stage of the Silurian. This monotonous succession of siltstones and mudstones is unique world-wide in providing an almost continuous faunal record of very shallow water conditions rather than an alternation of varying depth environments. The Eocoelia and related benthic animal communities persist from the early Llandovery through the entire Silurian and well into the early Gedinnian portion of the Lower Devonian. These rocks despite their relatively unmetamorphosed nature (never higher than the chlorite grade) have been highly folded and extensively faulted, beginning with disturbances associated with the Middle Devonian Acadian orogeny and extending into various intervals of the later Paleozoic and possibly Triassic. The complex structure is here outlined in more detail than is available in previous reports, thus giving both geologists and paleontologists a better framework with which to understand both faunal and geologic problems associated with the rocks. During the Gedinnian, the entire region became a site of nonmarine, Old Red Sandstone—type deposition, but there is no evidence for any stratigraphic break between the underlying marine and the overlying nonmarine beds. The Devonian vertebrates are similar to those of the Welsh Borderland of England. The rhyolitic and basaltic volcanics . . .

Purpose and Scope of the Investigation The Arisaig area (Fig. 1; Pl. 1) has been selected for detailed study because it provides the most continuous and best exposed sections of marine Silurian and early Lower Devonian rocks in the Appalachian Mountain system. The earliest fossiliferous rocks present are lower Llandovery in age, and it is probable that the whole of the Silurian is represented in the 4,000 ft of beds that make up the Arisaig Group; but the underlying Bears Brook Volcanic Group has no known fossils, and the Silurian rocks are faulted against the Ordovician Browns Mountain Group. It is not clear, therefore, how large a stratigraphic break, if any, is present at the base of the Arisaig Group. The marine Silurian succession passes without a break up into marine Lower Devonian; the system junction comes near the top of the Stonehouse Formation. The basal marine Devonian beds are followed conformably by the nonmarine Knoydart Formation (about 1,000 ft thick) which contains Lower Devonian (upper Downtonian to middle Dittonian) fish remains. Special studies have also been made of the Bears Brook Volcanic Group and of the structure of the area; repeated movements along the Hollow Fault have resulted in complex folding and faulting in the inland parts of the area. The village of Arisaig (Fig. 1) lies on Northumberland Strait, 15 mi northeast of Antigonish, Nova Scotia. It is now easily accessible from New Glasgow or Antigonish by Nova Scotia Route 45. The area studied extends along the coast from McAras Brook (3 mi southwest . . .

Topography and Superficial Deposits The shoreline northeast from Arisaig Point (Pl. 1) is controlled by the outcrop of the rocks of the Bears Brook Volcanic Group. Between the shore and the Hollow Fault, however, the physical features depend more on the distribution of glacial drift than on the structure of the Paleozoic rocks. The Hollow Fault, as its name suggests, lies along a valley, the base of which is never more than 400 ft above sea level. It is bordered to the southeast by a steep scarp rising to about 800 ft within 1,500 ft of the valley. This scarp marks the margin of a plateau underlain by Ordovician rocks of the Browns Mountain Group. A few hills, formed by monzonite intrusions, rise above the undulating surface of the plateau. Williams (1914, p. 41) and Goldthwait (1924, p. 28) describe the plateau as part of a widespread surface extending over the whole of the Maritime Provinces, the remains of this “Atlantic Upland” now being “preserved only on the more resistant rocks.” Goldthwait follows Daly (1901) in considering the plateau to have been planed off in Cretaceous times; the plateau is certainly post-Triassic, but it could be much younger. In describing the Arisaig region, Williams (1914, p. 44–49) divided the lowlands into “The Shore Front” and the “Carboniferous Lowland.” “The Shore Front” covers the area of Silurian and Devonian rocks around Arisaig, where hills reach a height of 750 ft west of McAras Brook (Pl. 1) and gradually descend northeastward to 650 ft near . . .

Description of Formations Browns Mountain Group The chloritized graywackes, slates, argillites, and hematite beds of this group occur on the southeast side of the Hollow Fault and were not examined closely during the present study. Williams (1914, p. 55) records the inarticulate brachiopod Obolus (Lingulobus) spissa from iron-ore beds and associated micaceous sandstones on the East Branch of Doctors Brook and north of the Little Hollow (which runs parallel to the Hollow three-quarters of a mile southeast of the West Branch of Doctors Brook). The same brachiopod occurs with the Arenig (Lower Ordovician) graptolite Didymograptus nitidus in the iron-ore beds of Belle Isle, Newfoundland (Williams, 1914, p. 55, footnote). The probability of an Ordovician age is strengthened by the fact that the Browns Mountain Group is intruded by monzonite stocks which do not intrude the Silurian, and further, because pebbles from these stocks occur in the Malignant Cove Formation; the Browns Mountain Group is therefore certainly pre-Silurian, and probably older than the Late Ordovician or lower Llandovery Bears Brook Volcanic Group. Browns Mountain conglomerates, with pebbles up to 1 cm, grits, and sandstones occur high on the east bank of Doctors Brook above stations L17 and L21 (Pls. 5A, 5B); and basic tuffs and serpentine are exposed in the Hollow Fault zone along the East Branch of Doctors Brook (stations U88 to U94, Pl. 5C). Other outcrops noted include grits on the West Branch of Doctors Brook 8,000 ft upstream from the junction of the East and West Branches, and epidotized basic rocks on the East . . .