MINERALS AND GEOLOGY OF CANADA. 189 
used for the latter purpose. To these materials must be added the 
native copper, native silver, silver glance, amethyst quartz, and sul- 
phate of baryta, contained in the veins which traverse the bottom 
rocks of the upper copper-bearing series of Lake Superior on the 
coast and islands of Thunder Bay, as at Prince’s Location west of 
Fort William,* &c.—always supposing the altered rocks in question 
to be really a portion of the Potsdam Group. 
The sandstones and conglomerates of this group are developed 
chiefly in the Counties of Beauharnois, Vandreuil, Two Mountains, 
and Berthier in Eastern Canada; and in those of Grenville, Leeds, 
Lanark, Renfrew, and Carleton in Canada West. A narrow belt 
occurs also to the west of the gneissoid ridge that crosses the St. 
Lawrence at the Thousand Isles. This belt runs through the Tuwn- 
ships of Pittsburg, Storrington, and Loughborough, and dies out a 
little to the west of Knowiton Lake. At these various localities 
the Potsdam beds lie in unconformable position on the upturned 
edges or between the foldings of the Laurentian rocks. Strata be- 
longing to the Potsdam Group have likewise been traced out, by 
the officers of the Geological Survey, on the north shore of the 
Straits of Belle Isle ; and the formation is also thought, on good evi- 
dence, to occur between the Mingan Islands and the adjacent coast. 
The thickness of the formation appears to vary from about forty feet 
or less, in some localities, to six or even seven hundred feet, in others. 
Interesting exposures occur more particularly at the following 
places :—-Loughborough, Eel, and Knowlton Lakes; north shore 
of the St. Lawrence, a mile or two below Brockville; north shore 
and islands of Charleston Lake (Townships of Lansdown and Escott, 
in Leeds County); vicinity of Beverly in the Township of Bastard ; 
Otty Lake, in Drummond Township, and surrounding district ; 
Townships of Nepean and Gloucester, in Carleton County; Lake 
St. Louis; Lake of Two Mountains; Point St. Anne and Point du 
Grand Detour, in Vaudreuil; Lachute, on the Riviére du Nord ; 
River St. Maurice (various parts, near the Cachée, &c.) ; and Hem- 
mingford Mountain in the Township of that name, on the border 
line of the Province.t The name of this group is derived from 
* When the earlier portions of this essay were printed, the upper copper-bearing rocks of 
Lake Superior had not been definitely separated from the underlying and greatly resembling 
Horonian series. This should be borne in mind with regard to the descriptions of certain 
minerals in Part II. 
+ Many interesting details and measurements in reference to these and other localities, 
will be found in the Revised Report on the Geology of Canada, issued by Sir William Logan 
and his colleasues. 
