The large group of islands on the southern and eastern side of Black Bay, and south-west of St. Ignace, consists of innumerable islets, separated from each other by the close intersection of the three systems of dykes, which appear more prominent and strongly marked in their features further west, in Isle Royale and Victoria Islands, and about Thunder Bay.

But besides these six clearly defined systems, there seem to be two more, or at least one other distinct system running due north-west and south-east, cutting at right angles through Spar Island, and reappearing, as I understand from verbal communications of Mr. Foster, further south upon Point Keewenaw. This system is perhaps the cause of the bearing of the shores between Keewenaw Bay and Dead River; also of the outlet of Lake Superior between Point Iroquois and Gros Cap along the river St. Mary, unless this eastern system of intersection be distinct from the more western one.

But however this may be, so much is plain;-that at least six distinct systems of dykes, with peculiar characteristic trap, forming parallel ridges in the same system, but varying, for different angles, between the different systems, intersect the northern shores of Lake Superior, and have probably cut up the whole tract of rock, over the space which is now filled by the lake, in such a way as to destroy its continuity; to produce depressions, and to have gradually created an excavation which now forms the lake, and thus to have given to it its present outline. This process of intersection, these successive injections of different materials, have evidently modified, at various epochs, the relative level of the lake and land, and probably also occasioned the modification which we notice in the deposition of the shore drift, and the successive amphitheatric terraces which border, at various heights, its shores.

A more minute analysis of the mineralogical character of these dykes would no doubt afford satisfactory evidence of their original independence, and perhaps lead, in connection with a fuller investigation of their intersections, to the means of ascertaining their relative age. But I became fully aware of the geological importance and independence of these different systems of dykes only during my return, after leaving the neighborhood of Thunder Cape, the ground where this part of the subject might be best studied, and

therefore I can now only call the attention of geologists to these facts, in the hope that they may, at some future time, be more fully investigated.

The whole range of rocks which constitutes the northern shore of Lake Superior is so extensively metamorphic, and so thoroughly injected in all directions by veins intersecting each other, that it is no easy task to analyze their relations; and for a full illustration of this subject, minute maps of well-selected localities are required, such as travelling geologists on an occasional visit can scarcely prepare. But I should be perfectly satisfied to see these hints more completely wrought by others, satisfied, as I am, to have shown, at least, how a minute investigation of the geological phenomena of a restricted locality may lead to a better understanding of the origin of the geographical features of a country.

But let me repeat that it were a great mistake to ascribe the present form of Lake Superior to any single geological event. Its position in the main is no doubt determined by a dislocation between the primitive range north and the sedimentary deposit south.

But the working out of the details of its present form is owing to a series of injections of trap dykes of different characters, traversing the older rocks, in various directions, which, from their mineralogical differences, have no doubt been produced at different successive periods.

The diversity of rocks which occur on Lake Superior is very great, and there are varieties observed there which seem to be peculiar to that district, presenting innumerable transitions from one to another, of which the Alps even do not present more extensive examples.

Of these we have new red sandstone passing into porphyries, into quartzites, granites, and gneiss, the metamorphism being more or less perfect, so that the stratification is sometimes still preserved, or passes gradually into absolutely massive rocks. Again, the dykes intersect other rocks almost without altering them, or the alterations in the immediate contact are so intense as to leave no precise lines of demarcation between the dyke and the injected rock. But here again, the phenomena are so complicated, that unless the illustration be accompanied by a very detailed map it were useless to enter into more minute descriptions.

The collections I have made of these rocks are sufficiently extensive to afford materials for such an illustration, and I may, perhaps, on another occasion, publish a more detailed account of the geological features of the northern shores, unless the expected publication of the geological survey of Canada by Mr. Logan, renders this essay superfluous.

I would here acknowledge the benefit I have derived in my investigations from the published reports of this survey, and also from the verbal communications of Mr. McLeod of Sault St. Marie. The rocks which occur on the northern shores are so characteristic that they cannot be mistaken, and even should the materials which I have collected not be published more in full, they will at all events afford to those who study the geological distribution of erratic boulders, valuable means of comparison, which will show that most of the erratics which occur in the northern parts of the United States are derived from the primitive range extending north of the lakes reaching along Canada and the United States to the Atlantic Ocean.

Among these rocks there is a variety of deep red felspar porphyry speckled with epidote, which, from its brilliant color, particularly attracts attention, and which occurs all along the northern shore from the Pic to Thunder Bay. This variety I have not observed farther east, and it may perhaps be taken as a guide to ascertain the range of erratics derived from the northern shore of Lake Superior.

XII.

GEOLOGICAL RELATIONS OF THE VARIOUS COPPER DEPOSITS OF LAKE SUPERIOR.

THE general distribution of the different copper ores in the region of Lake Superior, presents some facts which seem to me to have a direct bearing upon the theory of their origin. It is a very remarkable circumstance that the largest masses of native copper should occur upon Point Keewenaw, and that the non-metallic ores should be diffused at various distances from the central region where the largest masses of native metallic copper occur. The various sulphurets and carbonates are found on the northern shores and about Lake Huron, in far greater proportion, and over a wider extent, than anywhere nearer the metallic centre. The black oxide itself is found beyond the limits of the large metallic masses, and nearer to them than the other ores. I cannot help thinking that this particu lar distribution has direct reference to the manner in which these various copper ores were diffused in the country where they occur. They seem to me clearly to indicate that the native copper is all plutonic; that its larger masses were thrown up in a melted state; and that from the main fissure through which they have found their way, they spread in smaller injections at considerable distances; but upon the larger masses in the central focus, the surrounding rocks could have little influence. New chemical combinations could hardly be formed between so compact masses, presenting, in comparison with their bulk, a small surface for contact with other mineral substances capable of being chemically combined with the copper. But where, at a distance, the mass was diffused in smaller proportions into

innumerable minute fissures, and thus presented a comparatively large surface of contact with the surrounding rocks, there the most diversified combinations could be formed, and thus the various ores appear in this characteristic distribution. The relations which these ores bear to the rocks in which they are contained, sustain fully this view, and even the circumstance that the black oxide is found in the vicinity of the main masses, when the sulphurets and carbonates occur at greater distances from them, would show that this ore is the result of the oxidation of some portion of the large metallic masses exposed more directly to the influence of oxygen in the process of cooling. Indeed, the phenomena respecting the distribution of the copper about Lake Superior, in all their natural relations, answer so fully to this view, that the whole process might easily be reproduced artificially on a small scale; and it appears strange to me that so many doubts can still be expressed respecting the origin of the copper about Lake Superior, and that this great feature of the distribution of its various ores should have been so totally overlooked.