Sir W, E, Logan on the Quebec Group, Sfc, 205 



The break in the succession of life between the Chazy and 

 Birdseye, is not so great as that between the Calciferous and 

 Chazy. It is not yet quite certain that in Canada a single spe- 

 cies passes upwards in the latter case, while in the former, the 

 proportion which does so is about one-sixth. It seems to be in 

 accordance with this, that we have evidence of a somewhat sud- 

 den submergence for the introduction of the Birdseye and Black 

 River group, and a somewhat rapid accumulation of its deposits. 

 Where these rest upon the Huronian and Laurentian series, the 

 beds of contact are often composed of angular fragments of the 

 rock beneath, and it frequently happens that the surface on which 

 these beds rest, is rough and uneven, broken into sharp project- 

 ing ledges and deep fissures, which have been filled up and cov- 

 ered over by the deposits in question, before sufficient time had 

 elapsed to permit the asperities of the bottom to be worn to a 

 smooth surface. 



An instance in illustration of this occurs on the Snake Islands, 

 west of Lacloche in Lake Huron, where the Birdseye and Black 

 River group rests on the quartzites of the Huronian series, and 

 the lowest bed of the group is made up of angular fragments of 

 the quartzites cemented together by the fossiliferous limestone ; 

 there is another at Marmora, where the same group, supported 

 by Laurentian rocks, fills up deep angular cavities in the surface. 

 Dr. Dawson has pointed out a striking instance of the pheno- 

 mena at Hog Lake, in Huntingdon ; others occur at Sloat's 

 Lake in Loughborough and places adjacent, as well as at Kings- 

 ton Mills, and the same phenomena are observable in the neigh- 

 bourhood of Murray Bay. 



As an instance of the probably rapid slope with which the bottom 

 of the Lower Silurian sea descended from shallow to deep water 

 during the Potsdam period, in the neighbourhood of Quebec, we 

 see that the surface of the gneiss now supporting the Trenton 

 formation at the Falls of Montmorency, must have been as much 

 as YOOO feet above that under the Island of Orleans, where the 

 Quebec group makes its appearance, while the distance between 

 the two positions does not exceed a mile and a half ; this would 

 give a slope of nearly forty-five degrees, and perhaps it would 

 not be extravagant to take it as more or less typical of the slope 

 on the whole line to Alabama. 



As the black shales and limestones subordinate to the Pots- 

 dam and the deposits of the Quebec group accumulated, the 



