THE MARGINS OF AVALONIA

During Cambrian and earliest Ordovician times, Avalonia was an area fo rming an integral part of the huge Gondwanan continent, probably along the northern margin of Amazonia, until in early Ordovician (late Aren ig or Llanvirn) time it split off from Gondwana, leaving a widening Rh eic Ocean to its south. Today, its southern margin with Gondwana exten ds northeast from east of Cape God, Massachusetts, through Nova Scotia north of the Meguma terrane, and thence below sea level to the south of Newfoundland On the eastern side of the present Atlantic, the south ern margin may separate southwest Portugal from the rest of the Iberia n Peninsula; it can be traced eastwards with more certainty from the s outh Cornwall nappes to a line separating the Northern Phyllite Belt ( on the southern margin of the Rhenohercynian terrane) and the Mid-Germ an Crystalline High. There is no certain evidence of Avalonian crust t o the northeast of the Elbe Line. The northern margin of Avalonia exte nds westwards from south of Denmark to the British Isles, where it mer ges with the Iapetus Ocean suture between Scotland and England. Traced westwards, it crosses Ireland and reappears in northern Newfoundland to the east of New World Island, where it may follow the trace of the Dog Bay Line and the Cape Ray Fault. Recent work suggests that the nor thern margin of Avalonia may clip the northern tip of Cape Breton Isla nd in Nova Scotia, and then enter the North American mainland at the B ay of Chaleur; it may then be traced from north and west of the Popelo gan and Bronson Hill arcs to Long Island Sound near Newhaven, Connecti cut. The Cambrian to Devonian faunas reflect the history of Avalonia: initially they were purely Gondwanan but, as Ordovician time proceeded , more genera crossed firstly the Tornquist Ocean as it narrowed betwe en Avalonia and Baltica to close in latest Ordovician and early Siluri an times, and secondly the Iapetus Ocean, so that by the early Siluria n most of the benthic shelly faunas, apart from the ostracods, were th e same round the adjacent margins of all three palaeocontinents.