OF ARTS AND SCIENCES. 215 



According to Logan the Potsdam includes not only the Red Sand- 

 rock, but the Georgia slates and the St. Albans group ; an approxi- 

 mative thickness of from 3,700 to 4,000 feet. He also puts into it, 

 between S wanton and Ilighgate Falls, schists with lentils of magnesian 

 limestone of the Phillipsburgh group. It must be remembered that 

 the Red Sandrock rests in discordant stratification upon the schists of 

 Georgia, the group of St. Albans, and that of Phillipsburgh ; thus giv- 

 ing to the Potsdam, as described by Logan, an extension and a mixed 

 composition such as this group does not possess at Potsdam, nor any- 

 where in the State of New York, where it was created. 



The Quebec group comprehends the Phillipsburgh group and that of 

 St. Albans, beiiinnini; at Iliifh^ate Falls and rising towards the north. 

 As this group is found throughout the region his ma]) covers, below 

 the Red Sandrock or Potsdam, he is obliged to see along tlie whole 

 course of accidental contact a fault, recognized at certain points and 

 supposed at others, where he inscribes supposed fuulf. In the western 

 part no fault is seen, and he does not seek to explain wdiy his Potsdam 

 is there superposed upon the Quebec group. All the schists of Swan- 

 ton and Highgate Springs are called " Trenton." 



After a long and very detailed study of all the region represented 

 on this map, I could not find a fault anywhere. There is discordant 

 stratification between the Red Sandrock and all the other beds, which 

 are all older ; and I found neither Calciferous, Chazy, Trenton, nor 

 Utica slate. The result of these studies was given in my " Carte 

 Geologique des Bords du Lac Champlain entre Georgia (Vermont), 

 Chazy (New York), et Phillipsburgh (Canada)." (See Bulletin Soc. 

 Geol. de France, Tom. IX. Plate I., 1881, Paris.) 



With these two documents any practical geologist can go upon the 

 ground and form an opinion as to the value of these diflferent inves- 

 tigations. 



I have given more attention to this volume, " Geology of Canada, 

 1863," because Sir W. E. Logan, having recognized the value of the 

 observations of Emmons in his Letter to Barrande, 31 December, 

 1860, here tries to suppress the Taconic sysfejn; first by creating the 

 Quebec group, and then by giving to the Potsdam sandstone a much 

 greater vertical extension than it possesses at Potsdam itself, and, lastly, 

 by putting directly below and in contact with the Potsdam the Hu- 

 ronian series, created at the expense of the Louder Taconic. 



1863-1881. — The palaeontologists, while extending the primordial 

 fauna above and below the Georgia slates, were alwa3's met bv appa- 

 ritions of certain forms of the second fauna which disconcerted them ; 



