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ON THE PHYSIOGRAPHY AND GEOLOGY OF THE

ISLAND OF GRAND MANAN.

By Prof. L. W. BAILEY,

The Island of Grand Manan, near the entrance of the Bay of Fundy, though so long and so well-known for its picturesque scenery and from the richness of the surrounding waters as a fishing-ground for marine invertebrates, has received comparatively little attention at the hands of the geologist. Statements bearing more or less directly upon its geological structure have indeed appeared from time to time, but since the date of Dr. Gesner's first exploration of the island in 1838) no examinations with a special view to the determination of that structure have been made until quite recently. The most discordant views have in consequence been entertained with reference to the age of its rock formations. A visit, of some four days duration, made during the summer of 1870, in pursuance of duties connected with the Geological Survey of Canada, having enabled me to examine a considerable portion of the island and to compare its rocks with those recognized upon the main-land of New Brunswick, I propose to give here some of the conclusions at which I have arrived.

The general form of the island of Grand Manan is that of an irregular elongated oval, of which the greater diameter is about fifteen and the shorter about seven miles. Its surface, for purposes of description, may conveniently be divided into two distinct regions, contrasted equally in their physical and in their geological features. Of these the westerly and more extensive tract, embracing more than two-thirds of the main island, has the character of a somewhat elevated plateau, traversed in a direction parallel to its length by a series of minor ridges and depressions, and exposing upon the western shore, which is remarkably uniform and entirely free from islands, a series of bold bluffs, vary. ing from two to four hundred feet in elevation.* This plateau is for the most part well wooded (with birch, maple, beech, &c.,)

Among flowering plants observed on the island (August 22nd) were Asters and Solidagots of several species, Scutellaria galericulata, Potentilla fruticosa, Campanula rotundifolia, Epilobium angustifolium, Sedum rhodiola, &c.

except near the surfaces of exposed cliffs or upon rocky ledges which are often densely covered with a low growth of Juniper (Juniperus.)

The descent from this plateau to the lower lands which form the eastern side of the island, though less abrupt than that just alluded to, is nevertheless everywhere well defined, much of the last named region, including nearly all the settled portions of the island, being commonly not above a height of twenty or thirty feet above tide-level, and often much less.* This side of the island is further contrasted with that which forms its western half in its great irregularity of outline and in the numerous islands, of greater or less size, by which it is bordered. The many harbours which indent this shore afford a safe refuge to those engaged in the pursuit of fishing, an occupation to which the inhabitants of the island are almost solely devoted.

The first published observations on the geology of Grand Manan are those of Dr. Gesner, who in his first report to the legislature of New Brunswick (1838) describes at some length its general topographical and mineralogical features. The two regions above contrasted were recognized, and described as consisting, the one of trap and the other of slates (talcose, hornblendic and chloritic) and quartz rock, intersected by trappean dykes ; but beyond an allusion to the resemblance of the first named rocks in general aspect and in the contained minerals to those of Blomidon in Nova Scotia, no attempt at determining the age of either of these formations was made. In the geological map of Dr. Robb, which was for the most part based upon the observations of Dr. Gesner, the belt of rocks last mentioned is simply indicated as trappean, while those of the eastern coast are colored as of Cambrian age. From this time until the appearance of the second edition of the Acadian Geology of Dr. Dawson, no published references to the geology of Grand Manan appear to have been made. In an Appendix, however, to the last named work a summary of some observations bearing upon this subject is given by Prof. A. E. Verrill, who, though visiting the island chiefly for zoological purposes, had at the same time been able to devote some attention to its geological structure. The formations distinguished by Prof. Verrill, and described as being unconformable, correspond to the two belts recognized by Dr. Gesner, and to which allusion has already been made in describing the physical features of the island. That which forms its eastern side, and which was supposed to be the oldest, was found to consist of talcose and clay slates, mostly grayish, but sometimes black, cal careous grits, altered grey sandstones, the latter by induration sometimes becoming quartzites, or (when impure) imperfect syenites, and at some points black fissile carbonaceous shales ;the series, as a whole, being highly altered and disturbed, with numerous immense dykes and masses of trap. The sandstones in one case are described as containing vegetable traces. These rocks were found to occupy not only the belt of low land skirting the eastern border of the main island, but also as far as examined) the adjacent islands, excepting Inner Wood Island, composed in part of conglomerates and red sandstones, possibly of more recent origin, and the outer of the Three Islands, wherein were found beds of crystalline limestone.* The second series, embracing the trappean belt which forms the western side and the major portion of the main island, is described by Prof. Verrill as consisting of thick-bedded, regularly stratified massive rocks of various composition, but mostly amygdaloidal, trap ash, and compact quartzose rocks, the beds being in some places nearly horizontal, and in others dipping to the W. or S. W. > 10° to 20o. The traps at some points were found to be columnar, while from the cavities of the amygdaloids were obtained calcite, stilbite, apophyllite and other zeolitic minerals. With regard to the age of the two formations thus distinguished, Prof. Verrill makes no reference to that of the former beyond the statement that it is apparently the older of the two, but offers the conjecture that the latter, judging from the appearance of the rocks alone, may of Devonian age.

* An exception to this low and level character occurs at the northeastern end of the island, where the large peninsula separating Whale Cove and Flag's Cove is somewhat high and broken.

In commenting on these observations the author of the Acadian Geology thinks it probable that the outer and older series above mentioned may be either the equivalent of the St. John group (Primordial) or of the Kingston series (at that time supposed to be of Upper Silurian age), and that the traps, with some associated sandstones, might be Devonian or Upper Silurian. In the geological map accompanying this work these formations are re

be

• Observed also by Dr. Gesner.

presented in accordance with one of these conjectures, the one as of Lower and the other as of Upper Silurian age.

That the great belt of trappean rocks which form so marked a feature both in the physical structure and in the geology of Grand Manan, is of much more recent date than is supposed in the above observations, will, I think, with a full knowledge of the facts, scarcely admit of doubt. After a careful examination of a considerable part of their area, both as exposed in the shore cliffs and over the interior, I have no hesitation in re-affirming the comparison, long since made by Dr. Gesner, between these rocks and those of the North Mountains of Nova Scotia. So far as I have had an opportunity of examining the latter, their resemblance to those of Grand Manan is very striking, as well in their composition as in their general aspect, while both are quite unlike anything met with among the older recognized formations of New Brunswick. These traps at Grand Manan, though largely stratified, have evidently come up through the older metamorphic rocks of the island (which are at some points, as at the Swallow Tail Light, intersected by large dykes of exactly similar character), and were probably contemporaneous with the similar outflows at Blomidon and elsewhere, but whether the period of this eruption is to be assigned to the Triassic or to a still more recent epoch, is as yet undetermined. As tending to confirm the view of the Mesozoic

age

of these rocks, I was fortunate in being able to examine in situ the sandstones referred to, but not seen by Prof. Verrill, as sometimes occurring with them. These are rarely met with, (at least in that part of the island visited by me) being exceedingly soft and easily worn away except where protected by overlying masses of harder trap. They may, however, be seen near the entrance of Dark Harbor, the principal and almost the only break in the continuity of the western shore, and are said to be exposed at other points as well. In their features of softness and incoherence, as well as in their peculiar light red colour, these sandstones resemble very closely those of the Annapolis and Cornwallis valleys in Nova Scotia, or those which, at Quaco and elsewhere on the southern coast of New Brunswick, have been referred to the New Red Sandstone Era.*

• G. F. Matthew-Observations on the Geology of St. John County, N. B. Also, Bailey and Matthew-Observations on the Geology of Southern New Brunswick,

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