1. On the GEOLOGY of ARISAIG, NOVA SCOTIA.
By the Rev. D. HONEYMAN, F.G.S.

1. Introduction.

CONTENTS.

2. General Description of the Arisaig District.

3. Section from North to South.

4. Section from the East of Arisaig Pier to Doctor's Brook.
5. Section from the Frenchman's Barn to M'Cara's Brook.
6. Conclusion.

§ 1. Introduction.

THE district which is the subject of this paper is situated on the north-east side of Nova Scotia, on the Gulf of St. Lawrence, and from ten to fourteen miles south-west of Cape St. George. Several years since, this locality was brought under the notice of the Society by Dr. J. W. Dawson.

Its characteristic strata were then considered by him to be of Silurian age (Quart. Journ. Geol. Soc. vol. vi. p. 347). Subsequently in his Acadian Geology,' guided by the opinion of Sir Charles Lyell, he pronounced these deposits to be equivalent to the Hamilton and Chemung (U. S.) groups, and consequently to be of Devonian

age.

6

Having made the locality a special study for a great part of two years, and having compared the fossils obtained from the strata occurring there with those figured in Sir R. I. Murchison's Siluria,' I was convinced, beyond all doubt, that they were for the most part equivalent to the Upper Ludlow*. In the following year Dr. Dawson communicated a paper to the same Society, "On the Geology of Nova Scotia," in which, besides confirming my opinion, he separated the strata in question into an upper and a lower series†. Further progress was afterwards made by Professor Hall's determination of many of the Arisaig fossils.

Professor Hall and Dr. Dawson have again confirmed my opinion in regard to the age of the upper member of this series, pronouncing it to be the equivalent of the Lower Helderberg group, and the lower to be the equivalent of the Clinton, U. S.; for one of its characteristic organisms is a Graptolite, not distinguishable, according to Professor Hall, from Graptolithus Clintonensis, Hall.

Having examined another locality somewhat particularly, at the East River, I found, in situ, fossils similar to those of the upper member of the Arisaig series; and others, also in situ, which I considered were of a different age. I had found organisms similar to the latter, in boulders on the Arisaig shore, in abundance. I was therefore led to infer the existence of a member of the Arisaig series between the equivalents of the Upper Ludlow and the Clinton, which I considered to be equivalent to the Wenlock. This belief *See my paper "On the Fossiliferous Rocks of Arisaig," Trans. Lit. and Sci. Soc. Nova Scotia, 1859.

Trans., &c., 1860.

See Hall's Appendix to Dawson's Paper "On the Silurian and Devonian Rocks of Nova Scotia," Canadian Naturalist and Geologist, vol. v. pp. 144 et seq.

VOL. XX.-PART I.

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was confirmed by my examination of new localities, in one of which occurred Upper Ludlow and Lower Helderberg and Clinton strata, without the intermediate formation; in another occurred Upper Arisaig beds, and strata which had not been found in any of the other localities, but which I considered as possibly also of Wenlock age. In a paper on these new localities, read in 1860 before the Natural History Society of Montreal, this opinion was expressed by me. Dr. Dawson suggested that the new fossils were possibly Devonian*. In the same paper I announced the discovery of fossils in a part of the Arisaig series which was before considered as nonfossiliferous, and in a position considered by Dr. Dawson and myself as lower than the Clinton group. Still later I discovered at Arisaig, in situ, and in a position somewhat perplexing, fossils similar to those which I had considered as doubtfully Wenlock, and Dr. Dawson as possibly Devonian. Another perplexing circumstance was stated by Dr. Dawson, in the note referred to, namely, that my newly discovered localities tended to confirm an opinion that he had elsewhere expressed, to the effect that the Silurian and Devonian strata, of which the Arisaig series formed a part, had been thrown into synclinal and anticlinal folds on the formation of the metamorphic mountain-ranges, on the skirts of which the various Silurian and Devonian localities are situated. Regarding this opinion as correct, it appeared to me that the dip of the Arisaig series was in the wrong direction, and diminished in proportion as it receded from the mountain-range, with the possible exception of the last-discovered part of the series. Supposing this last to be the upper bed, as it must be if its equivalent at the lately discovered locality, Lochaber, were Devonian, it should overlie the uppermost instead of the lowest member of the Silurian series.

This was the state of matters relative to this admirable and typical section in our Nova-Scotian geology, when I was engaged to make a collection of the rocks and fossils of our province for the International Exhibition of 1862. Sir Roderick Murchison, at my request, very kindly asked Mr. Salter to examine it, who accordingly inspected my divisions of fossils, and, studiously avoiding all inquiry into the opinions already entertained, he unhesitatingly referred my Upper Ludlow to the Ludlow Tilestone, my Wenlock (?) to the Aymestry Limestone, and Hall and Dawson's Clinton to a repetition of the Ludlow Tilestone. He could not decide on the fossils of No. 5 (?). With regard to No. 4, the equivalents of which had been regarded by Dr. Dawson as possibly Devonian, and by myself as possibly Wenlock, Mr. Salter at once referred them to the Mayhill Sandstone age, qualifying the whole with " approximately." The matter was thus, to me at least, cleared of doubt, except on one point where a difference of opinion existed, arising chiefly, as it appeared to me, from a difference of opinion regarding the Graptolite already referred to,-Hall considering it as Graptolithus Clintonensis, and Salter as G. Ludensis, the containing strata being accordingly considered by the one as equivalent to the Clinton group, and by the *See paper with Dawson's note, Canad. Nat. and Geol. vol. v. pp. 293 et seq.

other to the Ludlow Tilestone. So much light having been thrown upon the subject by Mr. Salter, the two uppermost and the lowest members of the series having been determined by him, it at once occurred to me that a thorough examination of the locality would now be sufficient to determine what still remained doubtful, and the results of that examination I now beg to lay before the Society.

§ 2. General Description of the Arisaig District.

The extreme length of the Silurian rocks at Arisaig is about three and a half miles, and their breadth three-quarters of a mile. The group is thoroughly representative of all the known Silurian localities in the eastern half of Nova Scotia, and, as far as I know, it is the only district in the eastern part of the province where every member of our Upper Silurian series exists *. Here we have the whole in an apparently uninterrupted succession, and easy of examination, much of it being exposed in vertical and horizontal sections, on a shore subject to violent storms, accumulations of ice, and other degrading agencies. It is also bounded and intersected to a considerable extent by streams, two of which rise in the elevated metamorphic range which bounds the group on the south. The general position of the Silurian strata here is that of a synclinal axis, although it appears to be altogether different from any of Dr. Dawson's synclinal and anticlinal folds. But I do not mean to say that an anticlinal never skirted the mountains that form the great southern boundary of our Silurian group. I would rather suppose that such had existed, and that it had been removed by denudation during the early part of the Carboniferous period. Possibly the detrital matter resulting from that process formed the Lower Carboniferous conglomerate which we find associated with limestone (D) at Doctor's Brook, and intervening between the said mountains and a considerable part of our Silurian strata. An examination of the natural section in the line of this stream, which proceeds, with its branches, from the mountains, and traverses the rear of the group, for fully one half of its length, in a deep valley, revealed no traces whatever of the supposed old anticlinal axis.

The formation of the present synclinal axis is evidently of still later date, these strata having been elevated and thrown into a synclinal fold by the augitic trap (a and a', fig. 1), which has also altered and upheaved the adjoining Lower Carboniferous conglomerate and limestone.

The trap (a) on the north, or shore, side of the group extends in an almost continuous line from Black Point to Arisaig Pier. There is one interruption where a cove has been formed, and where the strata have been left comparatively unaltered. The trap-rocks may

* In England it is very common for the Llandovery group to be absent; but the rarest of all cases is to find the Wenlock (or Niagara) group missing. It is with us a thicker series than the Ludlow, and is often present where that is altogether absent. See Salter's Report on the Nova-Scotian Silurian collection. In the western part of the province, the equivalent of the Wenlock or Niagara Limestone exists, according to Hall and Dawson, at New Canaan. (Dawson's Paper, Canad. Nat. and Geol. vol. v. p. 139.)

Fig. 3.

Axis.

E

S

Harbour.

B

--45° 45'

Pier.

be traced for some distance into the sea, making the shore rough and dangerous. To the west of Arisaig Pier the line of trap is interrupted, and a small bay has been formed by the denudation of the strata. The strata on the shore are consequently inclined more or less in proportion to their distance from the pier. At a certain distance they are inclined at low angles, and in some cases they are horizontal. About a mile from the pier, the strata on the shore again give evidence of disturbance, at no great distance seaward, although the cause is not visible. At the extreme of the group, south-west of the pier, trap (amygdaloid) again appears in immediate contact with strata of the uppermost member of the series, upheaving and overlying the strata at the point of contact. It again appears at intervals, constituting the shore-boundary of the Carboniferous formation: the trap (a') on the opposite side of the group appears between the mountain-range and the intervening Carboniferous conglomerate and limestone, and again between a part of the latter and our Silurian strata. I also observed it at other points on this side, between the strata and the mountain-range, as indicated in the Map. At varying distances from these intruded rocks I found the strata particularly disturbed, so that the group may be regarded, on the whole, as synclinal, the direction of the axis being very irregular and generally obscure; but the axis is seen approximately near the mouth of Doctor's Brook, at Arisaig Brook, and M'Adam's Brook. I purpose now to illustrate the subject more fully by means of three sections.

§ 3. Section from North to South.

The first section (fig. 2) intersects the district from north to south, at about one mile from its eastern side. The trap (a) of the north of the section is well exposed along the shore, and forms an interesting study. It is often vesicular, amygdaloidal, and porphyritic, and is frequently associated with tufa and tufaceous conglomerate, and interesting alterations of the sedimentary strata are seen at its various exposed points of contact with them. These sedimentary rocks lose all semblance of stratification, being sometimes hardly distinguishable from the trap itself, sometimes assuming a somewhat granitoid aspect, at others jaspideous, or beautifully turbinated, as if replete with organisms, in every case signs of organic existence being thoroughly obliterated.

The upper part of the bed A (fig. 2) has an interest of a different kind; it lies in the cove opposite the break in the line of trap, to which I have already referred, and, although tilted, the stratification is preserved, and contains many organisms. The lower part contains several patches of fossiliferous limestone; the vertical sections of these are generally plano-convex, with a maximum thickness of six inches. This limestone is very much hardened, and contains throughout beautiful crystals of iron-pyrites. These strata, which are about 200 feet thick, have, in addition to the fossils found in the beds just referred to, separate organisms, scattered abundantly through them from the bottom to the top. The fossils of this