No. 3.] ROBB — GEOLOGY OF ST. IGNACE ISLAND. 177 



connection with the nvnn object of my visit to the islancl, but 

 the details of these operations I do not propose to describe on 

 the present occasion. 



This dyke may have been filled, like the other crystalline trap 

 masses, from above ; but I am more inclined to the opinion that it 

 was injected or intruded from below, and may, in fact, have been 

 the vent from which the others were supplied ; for wliicli latter 

 opinion I have some special reasons which I shall submit further 

 on. There is another somewhat similar, but evidently much less 

 important dyke, occurring near our camping ground, and running 

 in an entirely different direction, but it is unnecessary now to do 

 more than merely mention its existence. 



Towards the southern end of the loc itim, and also in one of 

 the small outlying islands, red sandstone^, breccias and conglo- 

 merates appear to overly the amygd.iloid in sniill detached 

 patches ; but I have no doubt that these mark only the basset 

 edge or northern extremity of a great mass or stratum of the 

 same character, forming the bed of the 1 ike and extending inde- 

 finitely southwards under it. Associated with these at the locality 

 under notice occur enormous masses of purphyi itic ti'ap, to which, 

 as being a rather remarkable rock both for its scientific or 

 geognosic interest and for its beautiful appearance, especially 

 when polished, I desire to direct your speci il attention. Although 

 not apparently a bedded or i ven a jointed rock, it occurs inter- 

 laminated with the red sandstones, or at least distinctly overlying 

 them in regular planes of junction, cotiforming with the dip of 

 the sandstone, which is 30*^ to 40*^ to the south. It is also seen 

 at one ph>ce conspicuously to overlie the amygdaloid (which here 

 dips at the same higher angle) conformably at or near the water 

 level, at the base of a high beetling clilf of the porphyrj^, the signi- 

 ficance of which facts I shall presently j^roeeed to explain. The 

 same rock occupies uninterruptedly, for about two miles, almost 

 the etitire southern limits of the location, forming a succession of 

 bold headlands fronting on Lake Superior; and is succeeded ia 

 going west by the underlying red breccias. 



From these facts, I think it will undoubtedly be obvious that 

 the porphyry bt longs to, and is newer than the sandstones and 

 auiygdaloids lying to the north of it. 



Tiiese rocks I take to be the same as those described by Dr. 

 T. Sterry Hunt, in his able "Report on the Trap Dykes and 

 Azoic Kocks of South-Kastern Pennyslvauiaj 1878," which are 

 Vol. X. M No. 3. 



