140 ROYAL SOCIETY OF CANADA 



sedimentary deposits along the north side oi' the lower Ottawa at present 

 form a narrow margin along the stream which is continuons almost to 

 the Chats Falls, nearly one hundred and tifty miles from its mouth. The 

 area is bounded by ranges of hills which rise a short distance back from 

 the river ; but to the south of the Ottawa the country is largely a level 

 plain, and the sedimentary formations spread over a ver}' considerable 

 extent, whose western limit may be roughly bounded by a line drawn from 

 Arnprioi-, forty miles west of Ottawa city, to Brockville, on the St. 

 Lawrence, about one hundred and twenty-five miles west of Montreal, 

 Several small outcrops of the underlying crystalline rocks appear, how- 

 ever, from beneath these generally horizontal strata. 



The basal beds of this great series of fossiliferous sediments are fre- 

 quently composed of the debris of the Laurentian gneiss and limestone, 

 upon which these arkose strata rest, and these constitute the lowest 

 member of the Potsdam sandstone which is now regarded as the base of 

 Calciferous formation. The whole series along the loM'er Ottawa is of 

 special interest from the geological and pala^ontological standpoint, from 

 the fact that we have here, in unaltered form and undisturbed, a com 

 plete series of strata which ma}' be regarded as furnishing a typical 

 section of the Cambro-Silurian system from the very bottom to the over- 

 lying Silurian. While, however, several sections are found which enable 

 us to measure the thicknesses of the various formations, this thickness is 

 found to vary somewhat in ditferent portions of the area ; and though the 

 strata, as a whole, are comparatively liori/ontal in attitude the}' are at 

 many points alfected l)y local faults, some of which are of considerable 

 extent, so that the problem of the determination of the entire thickness 

 is thereby somewhat complicated. 



In addition to the Cambro-Silurian formations which occur in this 

 area, we find to the southeast of Ottawa city, in Osgoode and Eussell 

 townships, several outcroi:)S of a reddish sandy shale, precisely similar to 

 what have been described as occurring in the area east of the St. Law- 

 rence, between Montreal and Quebec, and which have there been regarded 

 as belonging to the Medina. Like the St. Lawrence outliers these newer 

 red beds also appear to rest unconformably upon the Lorraine or Utica 

 shales, while in the western part of the basin, on Lake Temiscaming, the 

 geological section is still further extended upward by the presence of 

 several outliers of Silurian fossiliferous strata which represent the Niagara 

 formation. It will thus be seen that the fossiliferous sediments of the old 

 Ottawa basin have a very wide range, and that this area affords especially 

 good facilities for the study of these early sedimentary strata. 



The Potsdam sandstone is readily recognized by its peculiar physical 

 features wherever it occurs. It fills up depressions in the underlying 

 crystalline rocks, is uniformly siliceous in composition and varies in colour 

 from a gray to a deep red, the latter tint being apparently due to the 



