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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 . . .

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