MIDDLE AND UPPEE ORDOVICIAN. 



197 



The St. Lawrence Valley east of the river between Montreal and Quebec is divided 

 by the St. I.awrence-Champlain thrust fault into two zones, the line of division, 

 according to Logan, ^^*^ passing through Quebec and curving thence to Lake Cham- 

 plain. Strata that occur southeast of this line differ in lithology and faunas from 

 those of probably equivalent age that are found northwest of it. The facies differ 

 across the thrust fault. Those of the northwestern zone (Ottawa, Montreal, Quebec) 

 may be described under the New York names cited above, but those of the 

 southeastern zone ("Eastern Townships") belong to the Sillery, Levis, and Quebec 

 formations (named in ascending order), which constitute the old Quebec group of 

 Logan and Billings. The Sillery (Cambrian) and Levis (Beekmantown or "Cal- 

 ciferous") are described in Chapter III (pp. 131-134). The Quebec formation 

 (restricted) is the equivalent of the upper Chazy and lower Trenton.^®''' ^^ 



In EUs's report on the geology of the area shown on the Three Rivers map 

 sheet ^" occurs the following table, which is based on correlations by Ami : 



It is convenient to consider the formations northeast of the St. Lawrence- 

 Champlain fault separately from those southeast of it. We first take up the 

 Ottawa basin. 



The Paleozoic rocks of the Ottawa Basin are bounded on the southwest by the 



Frontenac axis, an outcrop of pre-Cambrian rocks which extends across the 



St. Lawrence into New York and connects with the Adirondack mass; beyond 



this axis to the southwest along the upper St. Lawrence and Lake Ontario is 



another area of Ordovician strata, in which the section differs somewhat from 



that of the Ottawa Basin. Ami ^ contrasts them as follows: 



Ordovician east of the Frontenac axis. 



VII. Lorraine: Buff-weathering and dark siliceous shales and mudstones. 

 VI. Utica: Dark-brown and black bituminous shales and limestones at the base. 

 V. Trenton: Dark-gray impure and semicrystalline fossiliferous limestone. 

 IV. Black River: Heavy-bedded and hard compact fine-grained impure limestones, etc. 

 III. Chazy: Limestones, shales, sandstones, and grits; shallow water deposit at the base. 

 II. Beekmantown (Calciferous): Dark-gray impure magnesian limestone or dolomites, cavernous 

 and foasUiferous. 

 I. Potsdam: Light-yellow and rusty-colored sandstones and conglomerates; shore deposits resting 

 on the Archean crystallines. 



