Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/34

 stratified traps. These have generally been regarded as corresponding either to the Potsdam Sandstone (Upper Cambrian), or to some of the lowest members of the Lower Silurian ; but Mr. Robert Bell, of the Geological Survey of Canada, has recently put forth the opinion that they are truly of Triassic or Permian age. In the absence of any satisfactory palaeontological evidence, this question cannot be definitely settled ; but the available stratigraphical evidence would rather support the view of Sir William Logan, that the Upper Copper-bearing series is of Lower Silurian age.

The Lower Copper-bearing series is well exposed on the north shore of Thunder Bay, extending beyond the limits of the bay as far westwards as tbe mouth of the Pigeon river. The series consists in ascending order of: — 1. Green siliceous conglomerates containing pebbles of quartz, jasper, and slate; 2. Grey and black chert -bands, separated from one another by thin courses of dark grey dolomite ; 3. Black shales and flags, with associated hornblendic traps ; 4. Grey argillaceous sandstones and shales. No organic remains have hitherto been discovered in the Lower Copper-bearing rocks ; and the age of the series is therefore uncertain. It is most probable, however, that the group should be referred to the base of the Lower Silurian series. The entire thickness of the Lower Copper-bearing rocks is probably about 1500 feet ; and their general strike is from nearly E. and W. to nearly E.N.E. and S.W. The series is traversed by trap-dykes ; and there are also several well-marked interstratified traps.

The Lower Copper-bearing series acquires a special interest from the fact that it is penetrated by two sets of mineral veins, which promise to become of great importance from the quantity of silver which they contain. The majority of the lodes run along the strike of the beds, having a general E.N.E. and W.S.W. direction ; but there is also a set of transverse lodes, which have a direction more or less nearly north and south.

Of the north and south lodes, the most important is the now celebrated " Silver-Islet " vein, situated on an exceedingly small rocky islet immediately to the south of Thunder Cape. The vein has a width of from three to four feet, and the vein-stuff consists of quartz impregnated with native silver and galena. Picked specimens of the ore assay from £1000 to £2000 per ton; but this is, of course, exceptional. The mine has only been worked for about two years, but it has hitherto proved extraordinarily productive.

The most important of the second series of veins, namely those which run along the strike of the stratified rocks and have a general E. and W. direction, is the so-called " Shuniah vein." This large vein has been worked at several points along its course, and traverses the Lower Copper-bearing rocks of the north shore of Thunder Bay, running pretty nearly parallel to the shore, and at a distance from it of 1½- to 2 miles. None of the mines on this lode can be said to be as yet out of their infancy ; but the results already obtained are such as to warrant very sanguine expectations. The Shuniah Mine itself is situated about 3½ miles to the N.E. of Prince