810 SIR J. W. DAWSON ON THE EOZOIC AND PALEOZOIC 



upon and to remove in large slabs and boulders, piling these up on 

 banks, to constitute masses of conglomerate. This would bespeak a 

 cold ice-laden sea as that in which the Graptolites lived, and it may- 

 account for the survival in these areas of old Trilobitic genera which 

 were not represented in the warmer waters of the continental 

 plateau. This circumstance has perhaps some connexion with the 

 greater apparent survival of these in America as compared with 

 Europe, though I suspect that the observed appearances depend in 

 part upon collectors attributing species belonging to fragments of 

 older limestones to the Quebec group itself. 



The importance of the Quebec group of Logan is thus vindicated, 

 as representing widely spread local conditions and great lapse of 

 geological time ; and the prescient view which he entertained of it 

 may be indicated by the following extract from a note appended by 

 him to Murray's Eeport on Newfoundland in 1865 : — 



" The sediments which in the first part of the Silurian period were 

 deposited in the ocean surrounding the Laurentian and Huronian 

 nucleus of the present American continent, appear to have differed 

 considerably in difi'erent areas. Oscillations in this ancient land 

 permitted to be spread over its surface, when at times submerged, 

 that series of apparently conformable deposits which constitute the 

 New York system, ranging from the Potsdam to the Hudson Eiver 

 formation. But between the Potsdam and Chazy periods, a sudden 

 continental elevation, and subsequent gradual subsidence, allowed 

 the accumulation of a great series of intermediate deposits, which 

 are displayed in the Green Mountains on one side of the ancient 

 nucleus, and in the metalliferous rocks of Lake Superior on the 

 other, but which are necessarily absent in the intermediate region 

 of New York and central Canada. 



" At an early date in the Silurian period, a great dislocation 

 commenced along the south-eastern line of the ancient gneissic 

 continent, which gave rise to the division that now forms the western 

 and eastern basins. The western basin includes those strata which 

 extended over the surface of the submerged continent, together with 

 the Pre-Chazy rocks of Lake Superior, while the Lower Silurian 

 rocks of the eastern basin present only the Pre-Chazy formations, 

 unconformably overlaid, in parts, by Upper Silurian and Devonian 

 rocks. The group between the Potsdam and Chazy, in the eastern 

 basin, has been separated into three divisions, but these subdivisions 

 have not yet been defined in the western basin. In the western 

 basin the measures are comparatively flat and undisturbed ; while 

 in the eastern they are thrown into innumerable undulations, a vast 

 majority of which present anticlinal forms overtui'ned on the north- 

 western side. The general sinuous north-east and south-west axis 

 of these undulations is parallel with the great dislocation of the St. 

 Lawrence, and the undulations themselves are a part of those 

 belonging to the Appalachain chain of mountains. It is in the 

 western basin that we must look for the more regular succession of 

 the Silurian rocks, from the time of the Chazy, and in the eastern, 

 including Newfoundland, for that of those anterior to it." 



