CAMBEIAN AND LOWER ORDOVICIAN. 131 



The later reports of the Canadian Survey, particularly those by Ells, contain 

 many statements and descriptions relating to the Potsdam and "Calciferous.""** 



The Cambrian terranes of the region southeast of the St. Lawrence-Champ lain 

 fault were described by Logan as part of the "Quebec group." They are now 

 comprised under the terms Levis and Sillery. Their typical character in the vicin- 

 ity of Quebec is summarized by Ami in the table quoted on pages 202-203. 



The "Quebec group" was established by Logan, ®**^ who described the strata 

 in the vicinity of Quebec and gave a detailed section, which is here included as an 

 account of the rocks, though no longer accepted as a correct interpretation of the 

 succession : 



At the upper end of the island of Orleans, on the northwest side, between high and low 

 water marks, there are exposed about 500 feet of black graptolitic shales, such as belong to the 

 Utica and Hudson River formations, dipping southeast 50°; resting upon which is a series of 

 different strata, dipping in the same direction, and at the same angle with them; the contact 

 between the two beiag visible. After a short distance across the measures, undulations occur. 

 The effect of these on the distribution of the beds has been partially ascertained, round the 

 upper end, to the southeast front of the island, and for several miles along it. The following 

 is the sequence of the strata; the beds are, however, in some places so corrugated and broken 

 that the measurements here given can only be taken as rude approximations to the truth. 



Feet. 



1. Green calcareo-magnesian shale, weathering to a yellowish or reddish brown, interstratified 



with thin bands of purplish-gray argillaceous shale. Some of the magnesian shales are 

 nearly grass-green, and the surfaces of most of the green beds are marked with fucoid- 

 like forms of purplish gray; the green shales hold about 20 per cent of dolomite. The 

 mass is strong and offers considerable resistance to weathering influences 100 



2 . Gray argillaceous shale, much softer than the magnesian shale 100 



3. Gray limestone conglomerate; the rounded masses are chiefly of gray limestone; the matrix 



in many parts weathers to a brownish color and is probably dolomitic. Fossils occur, 

 some of them replaced by silica, but those as yet obtained in this locality are too obscure 

 to be determined; the band in some parts appears to break into lenticular patches 10 



4. Green yellow- weathering calcareo-magnesian shale with gray argillaceous bands of the same 



character as No. 1 ' 100 



5. Gray soft argillaceous shale 200 



6. Yellowish-gray dolomite, weathering orange-brown. It holds occasional masses of ash-gray 



limestone and in some parts of its thickness a multitude of pebbles of quartz as large as 

 peas, and becomes toward the top a dolomitic sandstone 70 



7. Gray fine soft argillaceous shale, with compound graptolites (Phylhgraptus typtis) about 



30 feet from the summit 170 



8. Gray limestone conglomerate; the matrix in some parts weathers to a reddish brown, being 



dolomitic, and contains large concretions of carbonate of lime in concentric fibrous layers 

 like travertine. The band holds fossils in some places 35 



9. Gray fine soft shale, with occasional bands of sandstone weathering browrdsh, none of them 



over 6 inches; the bands of sandstone increase in number toward the top 50O 



10. Olive-green argillaceous shale, striped with purplish-gray bands 700 



11. Olive-green arenaceous shale, with disseminated soft grains of a green mineral, resembling 



glauconite and approaching it in composition. In the upper part of tiie deposit the shale 

 contains so much grit as to become almost a sandstone; and within about 100 feet of the 

 top it assumes a red color, in one or two bands 400 



12. Yellowish-white limestone conglomerate; the matrix assumes a dolomitic aspect in some 



parts; the rounded masses or bowlders are occasionally 1 or 2 feet in diameter, and some 

 parts of the bed hold fossils 10 



13. Gray, drab-weathering sandstones, in general slightly calcareous, interstratified with gray 



argillaceous shales; some of the sandstone beds toward the bottom are 3 or 4 feet thick 

 and hold occasional calcareous pebbles. The sandstones become thinner ascending, and 

 then the shales prevail; but these become by degrees more and more arenaceous, and a 

 band or two about 200 feet from the top assumes a red color 400 



