402 CAMBRIAN AND SILURIAN IN NORTH AMERICA. [XV. 



necessary to draw a line from the St. Lawrence, near Quebec, 

 to the vicinity of Lake Champlain, separating the true Hud- 

 son Kiver group, with its overlying Oneida or Medina rocks, 

 on the northwest side, from the so-calJed Quebec group, on the 

 south and east. This division was by Logan ascribed to a con- 

 tinuous dislocation, which had disturbed a great conformable 

 palaeozoic series, including the whole of the members of the 

 New York system from the base of the Potsdam to the sum- 

 mit of the Hudson River group, and, throughout the whole 

 distance of one hundred and sixty miles, had raised up the 

 lower formations in a contorted and inclined attitude, and 

 caused them to overlie in many cases the higher formations of 

 the system. This dividing line was by Logan traced north- 

 eastward through the island of Orleans, the waters of the 

 lower St. Lawrence, and along the north shore of Gaspe ; and 

 southwestward through Vermont, across the Hudson, as far at 

 least as Virginia; separating, throughout, the rocks of the 

 Quebec and Potsdam groups, with their primordial fauna, from 

 those of the Trenton and Hudson River groups, with the second 

 fauna. This is shown in the geological map of eastern America 

 from Virginia to the St. Lawrence, which appears in the Atlas 

 to the Geology of Canada, published in 1865. In an earlier 

 geological map, published by Sir "William Logan at Paris in 

 1855, before this distinction had been drawn, the region in 

 question in eastern Canada is colored partly as the Oneida 

 formation, and partly as the Hudson River group; while in 

 the accompanying text the Sillery sandstone is spoken of as the 

 equivalent of the Shawangunk grit or Oneida conglomerate of 

 the New York system. (Esquisse Geologique du Canada. 

 Logan and Sterry Hunt : Paris, 1855, page 51.) These rocks 

 were by Logan traced southward across the frontier of Canada, 

 into Vermont, where they included the Red sand-rock and its 

 associated slates ; which were thus by Logan, as well as by 

 Adams, looked upon as occupying a position at the summit of 

 the second fauna. When, therefore, in 1859, Professor Hall 

 described the trilobites found in these slates in Georgia in Ver- 

 mont, he referred them to the genus Olenus, whose primordial 



