[ells] PALiEOZOIC OUTLIERS IN THE OTTAWA RIVER BASIN 141 



presence of a considerable percentage of iron (hematite) in its composi- 

 tion. Its observed thickness, on the Ottawa, is no where more than fifty 

 to sixty feet and in places is much less. In the upper stretches of the 

 river it is entirely wanting, the upper formations such as Calciferous, 

 Chazy or Trenton resting upon the Laurentian. In the development of 

 the Potsdam sandstone and the Calciferous iVjrmations, as studied at 

 many points, no line of separation is possible, the one passing into the 

 other by insensible gradations through the addition of calcareous matter 

 to the siliceous beds of the lower member. 



The typical Potsdam sandstone has not yet been recognized in the 

 Ottawa Eiver basin, much beyond the township of March or about twenty 

 miles west of Ottawa city. At the Chats Falls and further west along 

 the shore of the Chats Lake between Arnprior and the mouth of the 

 Bonnechère, the Calciferous forms the lowest member of the series and 

 fills up the inequalities in the Archaean floor. It is succeeded directly by 

 the greenish-gray shales and sandstones of the Chazy Avhich pass upward 

 by the development of calcareous bands into the upper or calcareous 

 portion of that formation. The line of the Chats Falls, properly speak- 

 ing, marks the western limit of the great lower Ottawa basin, though the 

 sedimentar}' beds further we.st, around the lower part of the Chats Lake, 

 w^ere probably at one time continuous. There is, however, a marked 

 bi'eak in the levels of the deposition of the Calciferous below and above 

 the falls ; the Chats Lake beds being at a considerably higher level than 

 those below. 



The great lower Ottawa basin is affected by several low undulations, 

 though the inclination of the strata is at a low angle throughout the area. 

 The lowest or Potsdam sandstone member is very regularly exposed along 

 the western or Ontario margin, the highest members, viz., the Lorraine 

 shales and the overlying Medina, being found nearer the northwestern 

 angle of the basin but a short distance to the south and east of Ottawa 

 city. 



Between the Calciferous and the Chaz^' a somewhat well defined 

 change in the character of the strata is visible at various points. Thus 

 the entire series of the former consists of limestones, somewhat siliceous, 

 but generally highly dolomitic, with a well defined fauna. Occasionally 

 somewhat thin arenaceous but dolomitic shales appear in the upper 

 portion. A peculiar feature of the limestone, and one by which the 

 formation can be readily recognized, is the presence of géodes, holding- 

 yellowish -white calc-spar, though sometimes with quartz crvstals or 

 gypsum. This peculiarity is seen in the dolomites, from the most 

 westerly outcrop on Allumette Island in the Ottawa, as well as in the beds 

 «ast of the St. Lawrence ; and as a whole the strata composing this for- 

 mation present a marked uniformity in texture and composition through- 

 out their whole extent. 



