IV. The Physical Features and Geology of the Paleozoic Basin, between the Lower Ottawa and St. Lawrence Rivers.

By R. W. ELLS, LL.D.

(Read May 29, 1900.)

In a paper read before this Society in 1894, the distribution of the principal Palæozoic outliers in the Ottawa basin was given. Since that date, however, a large amount of field work has been done by the officers of the Geological Survey in the district, not only along the Ottawa river itself, but in the country between the lower Ottawa and the St. Lawrence rivers, and many additional facts, relating to the structure of the fossiliferous rocks in the area, have been obtained. Some of these new facts are of interest, as relating to certain points of structure, more particularly as regards the presence of faults and anticlines which affect the generally horizontal formations, which have not heretofore been pointed out. They also elucidate some features in connection with economic questions pertaining to portions of the area, more especially with reference to the probability of the occurrence of natural gas and oil, concerning which considerable inquiry has lately arisen.

In the area between the lower Ottawa and the St. Lawrence rivers, there is a widespread development of the lower Palæozoic formations. These range upward from the base of the Potsdam sandstone, which is seen at a number of points resting upon the denuded surface of the crystalline rocks of the Archæan system, to the Medina shales, which here represent the lowest member of the Silurian proper. While these are all affected by low undulations, they lie generally in the form of a broad synclinal basin, the northern margin of which is seen along the Ottawa river, where it is bounded by the great area of the crystalline rocks which cross that stream from the south about thirty-five miles west of Ottawa city.

The western margin of this basin is also defined by crystalline rocks such as gneiss, limestone and certain granites, which belong to the Hastings series. The eastern margin of these rocks extends in a somewhat irregular outline from the vicinity of Arnprior to the Rideau Lakes and thence to the St. Lawrence river near the city of Brockville. On the south the area of the Palæozoic rocks extends across the St. Lawrence into the state of New York, where there is a large development of the Calciferous limestone and the Potsdam sandstone, which also in many places are there seen to rest upon the crystalline rocks.