Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/367

 many of which possess considerable value in an economic point of view. The most important of these is Galena, extensive deposits of which may be traced for a considerable distance through the townships of Tudor and Lake. It is notable that the lead-ores thus enjoy a geographical distribution entirely distinct from that of the iron-ores. The Galena usually occurs in a gangue of calcareous spar, and forms veins or lodes coursing through the Laurentian Limestone or calcareous schists. Copper-ores have been found, as previously stated, but never in sufficient quantity to render their working remunerative. A small and unimportant deposit of antimonite, or sulphide of antimony, has been found in the township of Sheffield. Plumbago, of greater or less purity, is occasionally met with in the Laurentian limestones of the county, but has not hitherto been worked.

Finally, attention may be directed to two other minerals, which, although of no economic value, are of mineralogical interest as species that have not hitherto been described from this county. One of these is Rutile, or oxide of titanium, which I have found penetrating the quartz of Hog Lake in the form of stout prismatic crystals, striated longitudinally, and presenting a hair- brown colour and a strong lustre. The other mineral is Schorl, or black tourmaline, which occurs on lot fifteen of range four in Madoc, as a reticulated mass of slender prismatic crystals imbedded in quartz.

Discussion.

Prof. Ramsay inquired as to the proof of the existence of so large a boulder as one of five acres in extent. Under ordinary circumstances large boulders fell from higher rocks on to the surface of glaciers beneath, and were by them transported to the places where now found ; but the fall of such a mass seemed almost incredible. He suggested that possibly it might be a boss of the Lower Laurentian beds standing out through Silurian strata.

Mr. David Forbes stated that the results of his own examination of some of the specimens from the gold-mines of this district did not quite tally with those recorded in the paper, especially those of the rocks in the neighbourhood of the veins. He considered that the gold in Canada was confined to the veins.

Mr. Prestwich cited the discovery of a boulder between Grantham and Peterborough, which was at least 400 feet in length, and consisted of a mass of Great Oolite.

Mr. Searles Wood mentioned a boulder of marl in the coast- section near Cromer upwards of 300 yards in length, and 60 feet in height.

Mr. Wallbridge, in reply, stated that the rock must have come at the least twenty miles from its original home. The surface of the Trenton limestone rock in the neighbourhood was striated in the direction of the boulder. There was no evidence of intrusion. The mass was traversed in two or three places by crevices.