430 THE CANADIAN NATURALIST. [Vol. vi. 



formation. The first account of this chano-e in the stratisra- 

 phical views of Logan occurs in his letter to Barrande, dated 

 December 31st, 1860. [Amer. Jour. Sci. II, xxxi, 216.] 



This important distinction once established, it was found ne- 

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

 vicinity of Lake Champlain, separating the true Hudson-River 

 group, with its overlying Oneida or Medina rocks, on the north- 

 west side, from the so-called Quebec group, on the south and 

 east. This division was by Logan ascribed to a continuous dis- 

 location, which had disturbed a great conformable paleozoic 

 series, including the whole of the members of the New York 

 system from the base of the Potsdam to the summit of the 

 Hudson-River group, and, throughout the whole distance of 160 

 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 

 Gasp^ ; and south-westward through Vermont, across the Hud- 

 son, 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 Jthe 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 conglo- 

 merate of the New York system. [Esquisse Geologique du 

 Canada; Logan and Sterry Hunt, Paris, 1855, page 51.] These 

 rocks were by Logan traced southwards across the frontier of 

 Canada, into Vermont, where they included the red sandrock 

 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, Prof. Hall 

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

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

 horizon in Europe was then well determined, but in deference to 



