10 The Geolos:ical Struditre 



o' 



he a reddisTi tufa, with fragments of green trap. Another 

 (12) has a nugget of native copper, imbedded among calc 

 spar crystals, coating a cavity in partially decomposed greenish 

 trap. Another has a thin plate of native copper, running 

 through the centre of a narrow vein of red laumonite. Another 

 (-22) is a vein of calc spar and datholite in a reddish tufaceous 

 rock. The dathohte occupies the inteiior of the vein, with calc 

 spar on both sides, and contains crystalline granules of copper, 

 •with a little green carbonate of the metal. Another (19) has 

 plates of native copper, moulded into the crevices between crys- 

 tals of quartz and calc spar, in the cavities of a peculiar pseudo- 

 porphyritic rock, which may have been a mechanical aggregate 

 •of felspathic fragments and volcanic ash. Another has small 

 grains of copper attached to crystals of green prehnite, and 

 moulded into cavities left by botryoidal concretions of that min- 

 eral. In many specimens accompanying the above, vitreous and 

 purple sulphurets of copper and galena occur in associations pre- 

 cisely similar to those of the native copper. 



The age of the cupriferous rocks of Lake Superior has been a 

 subject of much discussion ; and my observations do not bear on 

 this subject any farther than to convince me that the rocks seen 

 at Maimanse underlie the red sandstone of Goulais Point and 

 Sault St. Marie. From the observations of Logan, Owen, Foster, 

 and other geologists, I have, however, little doubt that these last 

 are really the equivalents of the Potsdam or Calciferous sand- 

 stones ; and on this view I have proceeeded throughout this paper. 



The Maimanse rocks are assigned by Sir W. E. Logan to the 

 geological horizon of the beds of the Bruce Mine and other 

 places on the north shore of Georgian Bay, as a portion of his 

 ante-silurian Huronian system, which, in the last mentioned lo- 

 cality underlies uncomformably the lowest Silurian rocks. This 

 unconformability has not, so far as I am aware, been observed in 

 Lake Superior ; but I should not be disposed, in the case of for- 

 mations without fossils, and similar in their sedimentary beds, to 

 attach much importance to it ; unless indeed it can be proved, 

 that the red sandstone of Sault St. Marie is of much later date, 

 and. has been formed out of the waste of the older Maimanse 

 rocks. The appearances rather indicate a great continuous series 

 of sedimentary and volcanic rocks, in some places presenting only 

 fragmentary debris, in others intermixed with volcanic ejections ; 

 and perhaps locally broken up by volcanic action, before the 

 close of the process of deposition. 



