SUB-DIVISION OF THE LAURENTIAN ROCKS OF CANADA,

Hitherto these ancient and highly metamorphic rocks have been regarded by geologists as an inextricable mass of confusion. Sir W. E. Logan has set himself to unravelling their intricacies, with a patient perseverance of which hardly any other geologist is capable. A summary of his results was presented in the following manner :

"I have already indicated the probable separation of the Laurentian rocks of Canada into two great groups: that characterized by the presence of much lime and that without; but from recent investigation, the result of which has just been reported to the Canadian Government, it appears to me almost certain that the former of these two great groups will be capable of subdivision, and that some of its bands of limestone, with their associated strata, are of sufficient importance to be represented separately on the map. Having followed out one of these bands of limestone through all its windings, for a distance of eighty miles, the object of the present paper is to exhibit to the Section its geographical distribution, and the forms it presents in the physical structure of the region which it characterises. What at first appear to be two bands of these limestones, emerge from beneath the Lower Siurian series in the township of Grenville, on the Ottawa, and run into the interior parallel to one another, striking N. N. E. They are about two miles separated from one another, and both, with the gneiss between, dip in one direction, which is N.N.W, at angles varying from about 50 to 70 degrees. Attaining the rear of the township. a distance of about ten miles, the two bands unite, and are found really to constitute but one, the thickness of which, as far as I can make it out, is from 500 to 1,000 feet. It is plain from this distribution that the limestone is part of the out crop of an undulating sheet, the ridges of which have been worn down. But in the horizontal section of an undulating surface, similar forms in the distribution of the rim, may be derived from the anticlinal or synclinal part of the undulation, and as the dips on the opposite sides are both one way, it is a question to which part the area belongs. Within a short distance of the eastern side of the limestone, in fact, touching it in one place, an intrusive syenite makes its appearance belonging to a mass which occupies about 30 square miles in the townships of Grenville and Chatham, and runs to a point in Wentworth. The intrusion of such a mass of igneous rock as this can scarcely fail to have had a considerable effect in modifying the

attitude of the strata which surround it. The crystalline condition of the syenite shews that it was slowly cooled under great pressure, and we cannot now say whether it was a deep-seated part of an outburst which reached the surface, as it was then constituted, or whether it was originally overlaid by masses of gneiss and limestone, which have since been worn away. In either case the probability is, that it would give to the strata now surrounding it, an anticlinal form. It seems probable, therefore, that the western dip, belonging to the eastern band of limestone, where it approaches the syenite, is a true one, and that the form between the bands is synclinal. This appears to be corroborated by the fact that where transverse valleys occur between them, the wearing down of the intermediate gneiss widens the calcareous bands, particularly the east one, and narrows the interval.

The calcareous sheet having thus the form of a trough, the western dip of the western out-crop must be an over turn; and two spurs of the rock which point to one another, the one turning south from the western belt, and the other north from the eastern, must constitute a subordinate antielinal. Without reference to minor corrugations, the general form of the area would be that of two troughs joined together, each about a mile and a half wide, with an overturn dip on the west side, the one trough running north and south, and the other, as far as unconcealed by the superior fossiliferous strata, southsouth-west and north-north-east. The opposite sides of this calcareous trough run into two valleys, which unite at its northern extremity. But though the limestone then crops out, the valley continues northward into Harrington, and after a short interval shows an isolated patch of limestone of about a mile and a half in length, by a mile in breath, possessing, of course, a synclinal form. Beyond this, the valley splits into two, and while one branch runs rather north of N. E., the other turns N. of E. Each of these valleys is paved with limestone, the distribution of which shews a continuation of the synclinal form, with a bend more to the eastward than before.

The calcareous band on the western side has been traced to the north boundary of the township of Harrington, whence it crosses into Montcalm. It there appears to turn to the westward, but it has not yet been farther accurately examined. The eastern branch has been followed for between six and seven miles into Wentworth, when it appears to turn upon an anticlinal

axis, and proceeding in a bearing S. S. W., for seven miles, it attains the southern boundary of the township, close upon the east side of the northern prolongation of the intrusive syenite. It runs in the same bearing for about 3 miles along the eastern side of this into Chatham, and becomes deflected to the S. E. by the main body of the syenite, to which it runs parallel for about three miles. It then folds upon the axis of a synclinal, and running N. N. E. for upwards of five miles, returns into Wentworth, where it gradually bends round more to the eastward, and in about five miles reaches a position in the Gore of Chatham. It here folds over upon the axis of an anticlinal, and turning S. S. E. it maintains this course for about eight miles, in which it crosses into the Seigniory of Argenteuil and reaches the vicinity of Lachute, where it once more bends upon a synclinal axis, and proceeding eastward for about a mile, plunges under the Potsdam Sandstone and is lost.

In the winding course derived from the plications of the strata, the limestone usually presents a valley on the geographical surface; but to the west of all the folds that have been described, a bold ridge of gneiss runs from the front of Grenville to the rear of Harrington, the distance being about 20 miles and the bearing N. N. E. On the west side of this ridge, about midway of the length, there are two areas about five miles long and broad, presenting the forms of valleys, which are underlaid by limestone, so distributed as to render it probable that they are two outlying parallel troughs joined together, belonging to the same calcareous sheet as the one described. There would thus be four main synclinals and three main anticlinals, and the breadth they occupy altogether is about eighteeen miles, giving about four and a-half miles for the breadth of each undulation.

Bands of dolomite sometimes accompany, the limestone, which is often interstratified with bands of quartzite. The quartzite appears to be heaviest near the junction of the limestone and gneiss, becoming thinner and less frequent as we recede from the calcareous rock. The greatest mass of quartzite met with, had a vertical measure of 400 feet, and it was in stratigraphical position beneath the limestone. The quartzite and the gneiss on each side of the limestone are often very thickly studded with garnets, and in some cases the aggregation of these is so close as to constitute a granular garnet rock. In the Gore of Chatham a band of limestone about three-fourths of a mile to the

north-west of the one described, has been traced running paralle with it for seven miles. If the form which has been attributed to the first band be correct, the second would overlie it, with a great mass of gneiss between. A third band of limestone occurs about six miles north of the second; this has been traced for about four miles running east, which would be nearly parallel with the bearing of the second. In this bearing it has not yet been followed farther than to within a short distance from the line between the Seigniory of Argenteuil and the township of Abercrombie, towards the rear of both.

Continuous exposures of limestone have been met with on the west side of the Rivière du Nord at St. Jerome. They have been followed for two miles with a north bearing, and the strike of the stratification between Saint Jerome and the rear of Abercrombie, is such as to make it probable that the St. Jerome rock will ultimately prove to be a part of the third band. A feature common to both localities is the occurrence immediately near the limestone, of immense masses of lime feldspar. North of the Argenteuil band, eight miles, examined across the stratification, consist almost entirely of it, in the form of labradorite, of which masses of the opalescent variety are in some parts enclosed in a paste of the mineral without any play of colors, these feldspars are accompanied with hypersthene and ilmenite. This felspar rock is abundant at St. Jerome, and its stratified character is conspicuously displayed, the beds running parallel with the limestone.

Mr. Hunt has traced a band of crystalline limestone for eleven miles, running diagonally across the township of Rawdon in a north bearing. On the west side of this, lime-feldspar forms the great bulk of the rock exposures for twelve miles across the measures, and shows a well-marked stratification. It appears

probable that the Rawdon calcareous band is the same as the St. Jerome band, and that a synclinal axis exists between the two, the turn of the calcareous band on which is covered up by the fossiliferous rocks to the south.

In Chateau Richer below Quebec, a band of limestones occurs about a mile from the fossiliferous deposits, and to the northwest of it lime-feldspars present, a breadth of eight miles. On an island near Parry's Sound on Lake Huron, Dr. Bigsby observed the occurrence in situ of the opalescent variety of labradorite, and the name of the mineral reminds us of the existence of the rock beyond the eastern end of the province. It thus appears probable

E

that a range of rock will be found winding irregularly from one end of the province to the other, of sufficient importance to authorise its representation by a distinct color on the map, and a distinct designation in geological nomenclature.”

THE DEPOSITION OF NATIVE METALS IN VEIN FISSURES, &C., BY

ELECTRO-CHEMICAL AGENCY.

Prof. Chapman, of University College, Toronto, brought forward at considerable length his views on the origin of native metals in vein fissures. These views are at variance with those of the majority of chemists and geologists, at least in their application to the copper deposits of this continent. We give, therefore, an extended, and we believe faithful statement of Prof. Chapman's views from the Gazette.

"From the known fact that solutions of various metallic salts may be decomposed by voltaic agency, and the metal obtained in the simple state, it has long been a favorite theory with many geologists, that depositions of native metals, in veins, &c., are due to a similar cause. That such may be a perfectly legitimate conclusion in many instances, I am quite ready to admit; but, in applying this view to any particular case, it is necessary, unless the explanation is to be regarded as a mere theory of convenience, that certain collateral circumstances be not altogether excluded from consideration. If these circumstances oppose themselves to our theory, and remain by it altogether unanswered; nay, if but a single well-proved fact withhold its concurrence from the conditions demanded-surely it is more consistent with our obligations to scientific truth, that we abandon the theory at oncehowever plausible in itself, and however convenient in its application rather than attempt to maintain it by keeping these opposing conditions out of sight, or by wilfully ignoring their value. Now, my object in the present brief communication, is simply to bring before the notice of the Section, certain facts, experimental and otherwise, which appear to me to prove most incontestibly, that, in nine cases out of ten, the so-called electro-chemical theory as explanatory of the organ of native metals in veins, is entirely fallacious.

We will take the case of native copper, under its known conditions of occurrence in the Lake Superior District and other parts of North America. The electro-chemical theory is constantly being brought forward in explanation of this particular