Correspondence. 475 



but this I presume would scarcely be applicable to so many 

 identities in a fauna of such an aspect. Since there must be a 

 break, it will not be very difficult to point out its course and 

 its character. The whole Quebec group, from the base of the 

 magnesian conglomerates and their accompanying magnesian shales 

 to the summit of the Sillery sandstones, must have a thickness of 

 perhaps some 5000 or 7000 feet. It appears to be a great 

 development of strata about the horizon of the Chazy and 

 Calciferous, and it is brought to the surface by an overturn 

 anticlinal fold with a crack and a great dislocation running 

 along the summit, by which the Quebec group is ])i()aght to 

 overlap the Hudson River formation. Sometimes it may overlie 

 the overturned Utica formation, and in Vermont points of the 

 overturned Trenton appear occasionally to emerge from beneath 

 the overlap. 



A series of such dislocations traverses eastern North America 

 from Alabama to Canada. They have been described by Mes- 

 sieurs Rogers, and by Mr. Safford. The one in question comes 

 upon the boundary of the Province not over a couple of miles 

 from Lake Champlain. From this it proceeds in a gently curving 

 line to Quebec, keeping just north of the fortress ; thence it coasts 

 the north side of the Island of Orleans, leaving a narrow margin 

 on the island for the Hudson River or Utica formation. From 

 near the east end of the island it keeps under the waters of 

 the St. Lawrence to within eighty miles of the extremity of 

 Gasp^. Here again it leaves a strip of the Hudson River or 

 Utica formation on the coast. 



To the south-east of this line the Quebec group is arranged in long 

 narrow parallel synclinal forms with many overturn dips. These 

 synclinal forms are separated from one another on the main antic- 

 linals by dark grey and even black shales and limestones. These 

 have heretofore been taken by me for shales and limestones of the 

 Hudson River formation, which they strongly resemble, but as they 

 separate^the synclinals of the Quebec group must now be considered 

 older. I am not prepared to say that the Potsdam deposit in its 

 typical form of a sandstone is anywhere largely developed above 

 these shales, where the shales are in greatest force. Neither am I 

 prepared to assert its absence, as there are in some places masses ot 

 granular quartzite, not far removed from the magnesian rocks of 

 the Quebec group, which require farther investigation ; but, from 

 finding wind-mark and ripple-mark on closely succeeding layers 



