Examiner in Chemistry and Geology, University of King's College, Windsor, NS.
From the Transactions of the Institute of Mining and Mechanical Engineers, Vol. XXIII, 1874
DURING the summer of 1873, the writer was for some time occupied in examining the district on the south side of St. George's Bay, believed to contain coal seams of economic value. The beautiful sections of the divisions of the lower carboniferous exhibited here, continually suggested comparisons with those of Nova Scotia and Cape Breton, and the following rough observations present some slight interest as descriptive of the still unknown eastern edge of the great Acadia Carboniferous region.
In a paper communicated to the Nova Scotia Institute of Natural Science last December, a general sketch of the district was given, and the localities of the more important mineral deposits named. The last report of the Geological Survey of the island, issued this spring, contains estimates of the value of the coal districts agreeing with this, and points to the ores of copper and lead as the most certain sources of revenue. Up to last year the district presented all the charm of an unexplored country, nothing being known beyond the report of Mr. Jukes, and the vague assertions of the fishermen who spoke confidently of coal-seams measured by the yard. The labours of Mr. Murray, the able director of the Government Survey, have given to the world a valuable topographical map of the shores of the bay and careful analyses of the various sections.
In order to understand clearly the connection between the carboniferous strata of Newfoundland, Nova Scotia, and New Brunswick, the student must go back to the time when the greater part of North America was a vast plain, with perhaps a few low hills showing above the shallow seas; the Appalachians were part of the coal-making region, and the marine limestones of the Rocky Mountains prove them to have been mainly under the sea. Near the course of the present St. Lawrence must have been the drainage of that period, for the back country of New York and Canada contained azoic rocks of moderate elevation. The sombre and fathomless valley of the Saguenay may then have been the shallow bed of a paloeozoic brook, while away to the south and east stretched the Silurian and Laurentian hills of Nova Scotia and Newfoundland.
The waters of this great eastern sea thundered against the ancient cliffs and gradually deposited immense beds of conglomerate. When emergence took place there spread from Maine to Newfoundland the marshes and quiet lakes of the coal-producing age. The action was continued till above the productive strata (6,000 feet thick in Pictou county), there came the barren upper measures, whose thickness is given by Dawson at 3,000 feet. The section of a bore-hole put down last summer 500 feet in these measures, gives little beyond gray sandstones, and bluish shales with fire-clay and bands of ironstone, two of which measured 1 foot 6 inches. The beds of this series crop again in Prince Edward's Island, but no attempts have yet been made to penetrate to the underlying productive measures.
The sea has renewed its dominion over a great part of this region; but its former continuity is proved by the patches of carboniferous rocks along Eastern and Northern Cape Breton, and the presence of marine limestones in the Magdalen Islands. Portions of the coal-producing measures are left like black patches on the rim of this wide basin, relics of a coal-field rivalling in extent that of the interior continental region, and even surpassing it in thickness and in the value of its coal seams.
For these reasons the carboniferous strata of Newfoundland are very interesting, as from it the conditions that governed the deposits on one side of the Acadia region are obtained, and another link is added to the connection of the coal-fields of Europe and America.
The lowest of the five divisions of the carboniferous rocks, adopted by Acadian geologists, frequently contains peculiar bituminous and calcareous shales, with fish remains and thin seams of coal, resting on beds of conglomerate and coarse sandstone. In some districts the bituminous shales disappear and conglomerates prevail. The formation is not invariably present; in the section given in page 170 it does not intervene between the marine limestones and the silurian rocks. At Hillsboro', in New Brunswick, the well-known Albertite vein is found in the bituminous shales of this horizon. In Pictou the bituminous Ganoid shales do not appear, and the conglomerates have undergone extensive metamorphism, so that the conditions of deposit can hardly be deciphered. Taking the various measures exposed on the middle Barrasois River, on the south side of St. George's Bay, as well adapted for showing the best section of the productive measures exposed in Newfoundland, see Plate XXXV. The lower carboniferous series are found completely occupied by conglomerates, very coarse in the lower beds, but growing finer towards the top. The beds appear frequently of enormous thickness, and at irregular intervals contain thin beds of coarse sandstone. The origin of these strata is clearly shown by the boulders they contain. Many of the larger boulders, from two to three feet in diameter, are evidently from the range of Laurentian hills to the south, for the abundant rounded fragments of magnetite are indistinguishable in appearance from that found in sitar on the flanks of these hills. Other boulders again contain silurian fossils, and mark a series of strata now probably buried under these measures.
The stern Laurentian hills, which have seen the dawn and aided the progress of so many periods in the stony history of the earth, still furnish material for new strata. The torrents which pour over these conglomerates sweep from the hills, boulders identical in appearance with those which ages ago were concreted into solid rock.
The fragments of magnetic ore are frequently visible in the conglomerates of the succeeding periods. The writer observed them scattered in the rocks of a district at least 50 miles 'square, and corresponding in extent to the range of hills to the south known to contain this mineral. This wide-spread distribution of the ore shows the gigantic scale on which the deposits were formed, and the amount of ore originally present, when the remains after the waste of ages are engaging the attention of capitalists.
These strata are much disturbed by faults which in some cases do not appear to affect the overlying beds of marl and limestone. The thickness of these conglomerates is calculated by the officers of the Geological Survey at 1,300 feet, probably an under estimate, but the faults and uniformity of the beds render accurate measurements very difficult.
The measures of this group rest conformably on the conglomerates where exposed on the south shore of the bay, and are magnificently developed. The most striking feature in this horizon is the gypsum, nowhere in the Acadia district more prominent than at the Codroy Rivers. From Hillsboro', in New Brunswick, to Cape Breton, it forms a white rim round the coal measures ; in some places it appears in lofty white cliffs, in other localities its course is marked by curious funnel-shaped pits. Its horizon is confined to the sedimentary limestones, from which it has evidently been derived.
The following section, from measurements by Mr. R. Brown, gives a general idea of the strata usually associated with it :
Feet. Inches
Mixed Red and Brown Shales 12 0
(Frequently micaceous and sometimes argillaceous.)
Concretionary Limestone 4 0
Soft Blue Clay 3 0
Strong Slatey Limestone 47 0
Soft Blue Marl, with.Gypsum near the bottom 32 0
Gypsum 8 0
Soft Green (or Red) Marl (frequently Concretionary) 3 0
Red Marl, with layers of Limestone 28 0
Coarse Limestone and layers of Red Shale 44 0
The shales are sometimes laminated and ripple-marked. The fossils of similar horizons in Cape Breton and Nova Scotia are also common here, embracing Conularia, several species of Spinifer, two of Rhynchonella, and several of Productus, with abundance of Crinoids.
The writer is not aware of similar beds of gypsum occuring in strata of the same age in England, and believes they are confined to the Acadia limestones. To account for these anomalous deposits, the theory has been advanced that during the formation of the sedimentary limestones springs of sulphuric acid were poured out from submarine volcanoes and formed huge beds of gypsum by expelling the carbon dioxide. The action must have been on the grandest scale, for the beds are frequently found over 100 feet in thickness, and at one place in North-Eastern Nova Scotia present a bold cliff seventy yards in height. The action was to great extent simultaneous, for they are found always about the middle series of the limestones, and the district affected by these disturbances extends from Hillsboro' to Cape Breton, and from the Magdalen Islands to St. George's Bay. As far as yet ascertained these outpourings do not appear to have been accompanied by any notable disturbance, for the higher beds follow in regular succession.
The impure earthy limestone of the Salina period in New York has in many places been converted to gypsum, and the action is still in progress. Sulphur springs abound there, one of which is described by Dr. Beck as over a mile long and 160 feet deep, showing the power of the agency concerned and the effects on the rocks below.
That the Acadia gypsum was produced under similar conditions is supported by several facts, among which may be mentioned the occurrence in the beds of bleached quartzile pebbles, as if the action of the acid had been spent on a mass of calcareous matter containing sand and gravel. The planes of stratification are also observed, but no signs of life beyond an occasional patch of black bituminous matter. The presence of masses of limestones and anhydrite imbedded in the gypsum is not easily accounted for, but as far as at present can be ascertained, does not suffice to disprove the usually accepted theory. Little, however, is yet known about these beautiful deposits, which are yearly becoming of increased economic value. Over 120,000 tons were exported last year, and sold in the States for about three dollars per ton.
Above the gypsum come calcareous sandstones, red and blue marls, beds of micaceous red shale, and impure limestone; in general characteristics resembling those of the southern border, with the exception of the red marls, which do not appear in beds of equal thickness.
The comparative absence of lamination (the mixture of calcareous matter with sand), and the paucity of vegetable remains, would show that these upper strata were mainly formed in still waters little affected by currents. Mr. Murray gives a thickness of 2,150 feet to the measures of this section. Sir W. Logan, in his section of the Foggins measures, gives the thickness of the carboniferous marine formation at 1,650 feet. He does not include the lower beds of limestone and shale, which would swell its dimensions to at least 2,500 feet. In Pictou it appears to attain a greater size, but no detailed examination has yet been made.
These measures exhibit the conventional characteristics of this series, being little beyond a mass of coarse sandstones with every variety of texture. The remains of plants, with nests and seamlets of coal, are abundant in some of the beds about the middle of the series. Sometimes the carbonised fragments of plants have been replaced by copper pyrites. Generally the beds are of a brown colour, with occasional bands of bluish and red shales. In certain of the beds are layers of pebbles of limestone and quartzite, and fragments of magnetic iron ore similar to that described on page 169, as derived from the Laurentian hills. It is very possible that these pebbles may have come from the underlying carboniferous rocks. None of the limestones, however, showed any fossil remains. Some of the beds were uniform for thicknesses of 20 to 30 feet, frequently passing by imperceptible gradations into finer false-bedded sandstones.
The thickness is probably not far removed from that assigned by Sir W. Logan to the equivalent strata at the Fogins mines, where he measured nearly 6,000 feet of grit rocks underlying the coal measures. This enormous thickness is startling at first, but there can be no doubt in this case, for every bed can be measured as it stands exposed in the cliffs, cut and kept clean by the swift tides of the Bay of Fundy. pro. fessor Dawson does not show any rocks of the millstone grit in his sections of the Pictou coal-field, but the researches of the Geological Survey of Canada have proved its presence on all sides of the productive measures, not, however, sufficiently exposed to furnish data for an estimate of its thickness.
It is largely developed in Cape Breton, and well exposed at the base of the productive measures. In crossing from the comparatively soft strata of the coal formation, we immediately find protruding everywhere the sharp, rough edges of the grit rocks, which cover the ground in angular masses, leaving scanty room for vegetation.
The greater part of the writer's time and attention was devoted to these measures, but slow progress could be made. The ground was covered either by dense underwood or swamps, through which no prospecting pits could be sunk. The following is a summary of the information obtained.
The point of transition to the productive measures could not be clearly ascertained, owing to the presence of heavy beds of drift. The measures first met for about 150 feet of vertical thickness, consisting of light gray sandstones, with occasional bands of a darker colour, and light, arenaceous shales, holding ferns and casts of trees. About eight miles from the shore the first seam was met. Its thickness is 3 feet 10 inches, with layers of shale in the top. It was bright with mineral charcoal; the amount of ash, from the pieces burnt in the camp-fire, was very large. Where exposed in the bank of the river it appeared to be nipped out by a fault, and could not be traced Above this come a series of coarse brown sandstones, with casts of calamites. These strata are much broken by faults, with a general course slightly to the north of east.
About 200 feet in ascending order above these sandstones is a 6 inch seam, overlaid by 6 feet of fine gray post containing ferns, and 2 feet of black shale. Above this the measures are concealed; but at about 50 feet, measured at right angles to the planes of deposit, is a seam of very bright coal, one foot six inches thick, resting on three inches of fire-clay, crowded with stigmaria rootlets.
For the next hundred yards the measures are disturbed by a heavy fault which is a downthrow going south. Close to the south side is a four feet seam pitching at a heavy angle with the following section:
Feet. Inches.
Gray Sandstone 6 0
Brown (coarse) 2 0
Light Gray Sandstone 0 6
Good Coal 2 9
Ironstone 0 3
Coarse Coal, with thin, shaly Bands 1 0
Sandstone, with Ferns, &c.
The measures now become flatter, and at a vertical distance of one. hundred feet is a three feet seam of beautifully clean coal. The underclay was four inches thick, resting on light gray sandstone, the same material forming the roof. On breaking the coal it was clean, and divided with a cubical fracture. No impurity was visible beyond a few thin films of calcium carbonate. An analysis gave
Volatile Matter 33.4%
Fixed Carbon 59.3%
Ash 7.3%
Still ascending the river no more exposures of consequence were found, till in about one and a half miles the millstone grit rocks crop again, and there is nothing to indicate coal beyond that point. At a distance of 16 miles from its mouth, the river is met rushing from the flanks of the great Laurentian plateau.
The section thus presents a narrow trough of coal measures about two miles wide, and of unknown length. A four feet seam crops on Robinson's Brook four miles to the eastward, probably on the same horizon as one of the above-mentioned beds. Still further to the east the sub-productive measures were traced by the officers of the Survey from the shores of the bay to the mountains. The extent of the coal-field to the west is unknown; and here the most valuable discoveries may be made, for the measures appear less disturbed as they recede from the heavy fault which crosses the district obliquely. This trouble renders any estimate of the thickness of these strata unsatisfactory. There would be about 600 feet of measures exposed on the north side of it.
The difficulty of transport (there being no roads within a hundred miles), and the impenetrable nature of the country, have hitherto deterred investigators. The indications of coal, though encouraging, hardly warrant the expenditure of large sums of money when equally remunerative speculations can be undertaken at less risk.
On the north shore of the bay are similar exposures of carboniferous rocks, and at one place a thin seam crops out in connection with coal strata. As yet, however, there has been no return for the labour of prospectors.
Other thin seams are said to occur, but if any opinion can be formed from the measures exposed on the Barrasois rivers there is doubt if they prove of workable size.
The writer will now compare briefly the above description of the carboniferous rocks as they occur in St. George's Bay with equivalent horizons in Nova Scotia, as, by this means, some light is thrown on the value of the coal formation of the former place.
After the formation of several beds of conglomerate at the base of the carboniferous, we find, in parts of New Brunswick, and near Windsor, in Nova Scotia, periods of slight submergence, allowing the formation of bituminous shales, with abundance of vegetable remains and thin beds of coal; whilst in Pictou, Cape Breton, and Newfoundland the whole series is occupied by conglomerates, to the almost total exclusion of such remains.
When the limestone period is reached, the conditions appear indentical over the whole area of the Acadia region. The measures of NorthEastern Nova Scotia, Cape Breton, and Newfoundland, are precisely similar in fossils and structure. With slight variations in thickness and in composition of the strata, owing to the nature of the underlying boundary rocks, the section of one district will answer for all. There is the formation of calcareous matter, and the simultaneous outbreak of volcanic acid springs converting masses of limestone into gypsum, and then a return to periods of quiet movement suitable for the deposition of the marl and shale.
During the millstone grit epoch the conditions in the Foggins district appear to resemble more those of St. George's Bay than of Pictou and Cape Breton. At the former place these strata are grouped under three sections by the eminent geologists who have explored them. The upper series of 2,000 feet consist of red shales and red and gray sandstones, containing no coal and few fossils. The middle group contains nine small coal-seams, and many thick, coarse sandstones of light colour. And the lower division consists principally of red shales, with sandstones and con glomerates. Its thickness is stated by Sir W. Logan at 650 feet. On comparison with the equivalent measures of Newfoundland, already described, we find a striking agreement in the presence of vegetable remains about the middle of the series. There are more shales at the Foggins and less evidence of the prevalence of strong shifting currents So marked on the eastern side of the region. In Pictou and Cape Breton, as far as is ascertained, the beds of this period present little beyond coarse sandstones and shales, usually of a dark colour.
When the productive or middle coal measures come under consideration, the conditions vary much within comparatively short distances. In New Brunswick there is a scanty section of these measures affording hardly any prospect of valuable seams. Mr. G. F. Matthew, in the last report of the Canadian Survey, gives the maximum thickness of the productive measures at 200 feet. On passing to the Foggins there are 1,269 feet of measures containing nothing beyond rudimentary and thin coal seams. Above this is a thickness of 3,404 feet, holding a large number of thin seams, several of which are workable. Continuing to the eastward, the beds of coal improve in thickness, and at Springhill, fifteen miles distant, there are eight seams, each varying in thickness from two feet eight inches to thirteen and a half feet, included in 1,200 feet of measures. Below this is a series of barren measures resembling those of the Foggins. There is, however, no evdence that these seams can be identified with any at present worked on the shore, and the probability is that local conditions favoured the accumulation of coal in certain places.
At Pictou, again, an equally great change is found. Here the lower coal measures are the most productive. The total thickness from the lowest to the highest seam is 4,321 feet. Below the Pictou main or great seam, there are 830 feet of strata, consisting of fine argillaceous sandstones and shades, holding 100 feet of coal, in seams of greater thickness than two feet eight inches. Above the main seam come 1,200 feet of black carbonaceous and arenaceous shales ; and then the sandstones appear closing in as the courses of the currents were gradually diverted over the quiet nook so long covered with dense foliage. The upper beds contain seams from 3 to 8 feet thick, with much white post, and bear considerable resemblance to the Cape Breton measures. If the 1,800 feet of strata above the highest group of seams not yet proved to contain workable seams be added, there are 6,120 feet of measures included in the productive series.
At Cape Breton there are a series of seams from three to nine feet thick, apparently well disseminated through measures little under 5,000 feet in depth.
On comparing the coal measures of St. George's Bay, they are found to correspond more closely with the depauperated beds of the Foggins than with the other districts. The first seam met on the Barrasois measured three feet ten inches in thickness ; but the writer is of opinion that its size was owing to the fault, and that its normal dimensions were much smaller. This would give a large extent of measurer, containing a few seams of small size, which is supported by the result of explorations on the north side of the bay.
The similarity of conditions between this district and the Foggins, the one being at the extreme east and the other on the west border of the Acadia region, is remarkable as far as the underlying millstone rocks and the band of comparatively barren measures are concerned.
The immediate increase in the thickness of the seams included in the measures south of the fault, and the fact of its being a downthrow, might show that higher strata were brought in of more consequence in an economic point of view. Should future research confirm this view, the field would be confined to the district between the fault and the first outcrop of the grit rocks, a width of one and a half miles, and gradually narrowing to the east as the fault kept bringing up lower measures to the north. This would allow, judging as carefully as possible from the angles of dip of the strata exposed, a thickness of about 2,000 feet.
Several of the English coal-fields furnish instances of similar deposits of unproductive strata beneath the coal measures. In Denbighshire they are 1,000 feet in thickness, and the Gannister beds of South Lancashire furnish a parallel case. These, and the facts noticed at the Foggins, hold out the hope that future researches will prove more encouraging than the prospects offered by the Geological Survey, or by the present trials.
The VICE-PRESIDENT said, the paper was a very interesting one, describing, as it did, a new country, so far at least as their information about the coal measures goes, and a cordial vote of thanks was given to the writer.