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STRUCTURAL GEOLOGY OF NORTH AMERICA 



associated volcanic varieties. Much of this complex may be of Ordovician age. 

 In the southwestern part of the province the Charlotte group is probably of Or- 

 dovician age. It is made up of two divisions, one known as the Dark Argillite, 

 the other as the Pale Argillite. The former lies unconformably below strata of 

 Silurian age and is composed of argillite, slate, quartzite, mica schist, gneiss, 

 and minor amounts of volcanic rocks. It is intruded by masses of granite and 

 gabbro. The Pale Argillite consists of argillite, sandstone, arkose, slate, and 

 mica schist. In the St. Stephen area the beds are apparently comformable with 

 and grade into those of Dark Argillite. On early maps the Pale Argillite was 

 classed as Devonian on account of the reported finding on Cox Brook, a tributary 

 of Magaguadavic River, of a Lepidodendron-like form. Later work has failed 

 to find any fossils whatever in these rocks. 



In the Thetford area, the Quebec group (Sillery and Levis) consists of black 

 slates with a basal conglomerate and some interbedded impure quartzite or 

 graywacke, overlying unconformably the Cambrian Caldwell group. In the 

 Beauceville region volcanic tuffs and flows are interbedded with the sediments, 

 and in places the series is so altered that it is difficult to distinguish the volcanic 

 from the sedimentary members. Still farther southwest, near Phillipsburg in the 

 Lake Champlain region, a thick series of fossiliferous Beekmantown sediments 

 consisting of shales and limestones overlies Upper Cambrian beds and is fol- 

 lowed by strata of Chazy of Middle Ordovician age. 



To the northeast of Levis, rocks consisting of red, green, gray, and black 

 slates, quartzites, and conglomerates form a belt in places 20 miles wide. These 

 beds have been correlated with the Sillery, but both younger and older strata 

 may be included. An interesting feature in these rocks is the presences of belts 

 of limestone conglomerates. These occur at various horizons in both the Sillery 

 and the Levis, forming bands from about a foot to more than 100 feet in thick- 

 ness. The pebbles and boulders consist of gray limestone, and weigh from less 

 than an ounce to many tons. Similar limestone conglomerates are found in New- 

 foundland to the northeast and Vermont to the southwest. They have been in- 

 terpreted as the result of local slipping and breaking up of limestone along the 

 sea bottom by earthquakes in a zone where faulting was prevalent. Another 

 feature of the Sillery is the occurrence of belts of quartzite, locally called the 

 Kamouraska formation. These belts are lenticular but extensive, and their thick- 

 ness varies greatly. 



Interbedded arkose and volcanic rocks of Ordovician age are known 

 in the Shickshock Mountains; and dark shales, limestones, conglomerates, 

 argillites, quartzose sandstone, and volcanic flows and tuffs occur to the 

 south on both sides of Chaleur Bay. 



Silurian System 



The best Silurian section in Nova Scotia is at Arisaig where 3800 feet of 

 highly fossiliferous sandstones and shales occur. The series is overlain by 



Lower Devonian beds, and it overlies a flow of rhyolite probably of 

 Lower Ordovician age. The faunas can be correlated better with British 

 than with American; even the resemblances with the Chaleur Bay Silurian 

 faunas are slight. 



On the north side of Chaleur Bay is probably the thickest marine 

 Middle Silurian succession in North America. At the top of the sequence 

 are volcanic flows interbedded with sediments, chiefly elastics, and flows 

 are present also in other formations farther down in the succession. A 

 total of 8427 feet or more of sedimentary rocks and 4626 feet of volcanic 

 rocks are present in the Black Cape area. 



In southern New Brunswick, on the Bay of Fundy, great quantities of 

 volcanic rocks, chiefly rhyolites and andesites, are interbedded with sedi- 

 ments. At Oak Bay a basal Silurian coarse conglomerate rests unconform- 

 ably on the dark argillite of the Charlotte group of Ordovician age. The 

 belt is a continuation of one extending from the Eastport area of Maine, 

 where a number of formations of Middle and Upper Silurian age occur. 



Devonian System 



Rocks of Lower Devonian age occur in Quebec, New Brunswick, and 

 Nova Scotia. Sedimentation at this time was accompanied by widespread 

 volcanism, and at the close of the epoch the main phase of the Acadian 

 orogeny took place. In the Middle Devonian, great thicknesses of clastic 

 sediments accumulated in the Gaspe Peninsula, and in Upper Devonian 

 time sedimentation progressed locally in the Chaleur Bay and Bay of 

 Fundy regions ( Alcock, 1947 ) . 



A well-known Lower Devonian section is at the eastern end of Gaspe 

 Peninsula, where about 2000 feet of limestone and limy shale beds have 

 been described. Within central Gaspe Lower Devonian shales and lime- 

 stones, associated with thick deposits of volcanic rocks, are widespread. 

 At the west end of the peninsula, shales and argillaceous limestones of the 

 same age are 2200 feet thick. 



The Lower Devonian rocks at Dalhousie consist of highly fossiliferous 

 marine sediments, volcanic flows, and tuffs, dikes, and volcanic rocks. 

 The principal Nova Scotian Lower Devonian section is southwest of 

 Arisaig, where fine-grained, red, arenaceous slates and gray sandstones 



