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outer or south-eastern border of this basin, commencing in the centre of the Island, the strata arching over from beneath the coal bearing formation, dip eastwardly. A cursory examination of this basin with some diagrams were made by the present writer in 1841.
One of the earliest impressions made on the traveller, who, in visiting this region, brings recollections of ordinary coal fields, is the primitive, or rather the metamorphic and disturbed geological character of the entire rock series, much of which is probably new to him under its changed aspect. In fact very few persons, in passing through this region, would conceive themselves in the midst of a coal formation at all.
That we might the sooner attain a correct understanding regarding these novel appearances, some transverse sections were constructed, and also a profile following the east shore of the island. The west coast, being flat and without cliffs, did not well admit of such a mode of illustration.
The first transverse section, crossing from Narraganset Bay on the west by Butts' Hill Fort, to Mount Hope Bay on the east, exhibits an uninterrupted basin-form arrangement of stratification, having coal beds cropping out on opposite margins. Pursuing his inquiry, the geological observer will no longer doubt but the whole group of strata, many hundred feet thick, constitutes an actual coal formation, although its separate members seem to have little resemblance to such rocks as usually comprise our coal fields.
Fig. 16. Transverse Section of the Portsmouth Anthracite Basin, Rhode Island, looking North. Narraganset
Mt. Hope Bay.
Based upon more than one mass of very coarse conglomerate, in some positions consisting of large round pebbles of white quartz and fragments of primitive rocks, and in others of oval slaty fragments, formed from subjacent schistose rocks, are countless strata of greenish talcose slates; upon and among the lower series of which are conformable seams of white quartz, occasionally three feet thick; or again a net work of smaller quartz veins from a few inches to a foot thick, traversing both the conglomerate and the talcose slates. Among these slates occur darker laminæ, and these contain distinct impressions of the usual coal plants. Passing from these, the predominant mass consists of talcose schist, among the divisions of which may often be obscurely traced magnificent casts in relief, of ferns, pecopteris, &c. But for these intelligible characters, one might imagine the schists were of much greater geological antiquity.
Following southward along the eastern shore, the cliffs, although not lofty, are sufficiently so, with the aid of the transverse ravines, to develope the structure of the adjacent country. In the course of two miles of the cliffs of the east coast, the conglomerate beds are six times thrown up, and as often descend below the tide level. Then occur a numerous suite of twisted and contorted schists, of grey laminated slates whose surfaces singularly resemble the grain of birds-eye maple; and again another series of
green, talcose, contorted schists, crowded with crystals of iron pyrites; crossed in every direction by innumerable veins of white quartz, and succeeded by compacter beds which almost possess the qualities of sandstone. Perpendicular upthrows and heaves, and again the reverse movements, divide the whole series into large and separate sections, rising above or sinking below, water level. The inclination of the respective masses is continually changing. To the rocks we have enumerated succeed a melange of metamorphic slates, of grey fissile beds, of conglomerates, quartz veins, and black shales; of veins and filons of asbestos, and of talcose laminated strata; undulating, fractured, contorted, inverted-in short, disposed with such absence of order and arrangement, as to defy the pen and pencil of the geologist to delineate.
Leaving the coast line at Clark's Mill and Creek, our second transverse section, of 31 miles, crosses over to the opposite or western shore. During half this space, the metamorphic rocks alone, to which we have alluded, and which are named Grauwacke in the state report, appear on the surface and dip to the eastward. The schists and coarse slates, the carboniferous shales, and the quartz veins, which here seem to be appropriate to the coal-bearing series, are again seen arranged in the basin form, stretching to the coast near Lawton's Valley. We only observed one bed of anthracite, whose immediate out-crop exhibited about 14 foot thick, increasing as it descended. How far to the south this trough extends we did not attempt to trace; but as thin seams of coal are seen among the modified rocks, on the coast east of Newport, it is probable this arrangement continues through the entire length of the island.
Returning to our first transverse section of the coal basin, near the parallel of the Portsmouth mines. Certainly, there are many features here presenting themselves that have no parallel in our ordinary secondary coal fields. Among these are the vast assemblage of talcose, waving slates; the veins and seams of asbestos, abundant even among the coal shales, and occasionally penetrating the anthracite itself; the quartz veins also in the coal; the unusual appearance of vegetable remains on these greenish-grey, schistose laminæ; the traversing veins of white crystalline quartz, and the plumbaginous nature of nearly all the out-crops of coal. All these characters might readily lead geologists to ally the series to the transition or primary rocks.
Details of the Coal Beds.—Yet perhaps these are entirely due to the metamorphic influence to which the whole group, in common with all others in the surrounding country, has been subjected. There are three coal seams proved on the western side, occurring at the distance of ninety feet from each other, and dipping, at an angle of 38° to the centre, but probably flattening in that direction. Towards their out-crops all the strata evince the effects of great pressure and squeezing ; producing corresponding irreguJarities in the thickness of the coal beds, such as will probably always render the working or productive results uncertain.
In some particulars there appear to be analogies between the talcose schists and accompanying beds of anthracite in Rhode Island, and the anthracite seams associated with strata of gneiss and talcose schists, formerly considered as primitive, at L'Oisans, in Dauphiny, and the Alps. M. A. Brongniart has declared that the coal vegetation of this formation is identical with that of the true coal measures. M. S. Gras confirms this view, and states further that these were sedimentary rocks, modified by subterranean emanations. In the Alps there are many proofs of the transformation of sedimentary into crystalline rocks, as high up as the coal measures.
The old Portsmouth Mines, towards the close of 1841, had been re-opened, and several new shafts had been commenced in their vicinity. As regards facilities of transportation, no position can be more convenient; for sloops and schooners can approach within one or two hundred yards of the mine. The quality of the coal is excellent; the demand for it increases every year, and it can readily be sold, as fast as it is possible to mine it. In 1842, the price for the large coal was $5.00 per ton, and half that sum for the finest or pea coal : terms which can be commanded nowhere else at the pit's mouth, in the United States. As a proof of the value even the smallest had acquired, the owners were screening over the refuse heaps, abandoned 20 years before; and were selling the coarsest at $2 to $3 per ton; the next size at $1.25, and mere dust 75 to 50 cts. The quality of the coal improves as the depth increases. As may be inferred from the geological condition of this region, the great drawback on the prospective value of coal undertakings any where within its limits, and on the confidence so essential to such operations, arises from the irregularity of the ground; making the thickness and the continuity of any one coal seam, a matter of extreme uncertainty, even for the space of a few feet. The roof of the main worked bed is tolerably regular, and consists of a good hard slate; but the floor undulates considerably, and of course affects the thickness of coal to a corresponding extent.
At one point here there were only eighteen inches of coal, between roof and floor; yet on advancing but a short distance we observed a thickness of fifteen or sixteen feet. Under these circumstances, it is difficult to assume an average. Dr. Jackson's estimate of three feet workable coal to each seam, through the entire basin, may be a safe one, but we would not like to be the purchasers on the basis of that calculation. In 1842, the slope or inclined plane of the main gangway down the crop of the vein, was three hundred feet. Lateral drifts, following the coal seam, showed about three yards thickness; but we subsequently learned that it had again contracted.
The seam lying above this had been commenced by other owners, as a colliery, in 1841. Its thickness was then six feet; both roof and floor were good and promising, being of clay slate, dark, tough, and regular. Many coal plants occur in the roof.
The plumbaginous character of the carbonaceous deposits throughout the entire range from Mansfield in Massachusetts to Newport in Rhode Island, * is not devoid of interest, either to the miner or the mineralogist. At Wrentham in Massachusetts are several seams of highly plumbaginous coal. At Mansfield also, Dr. Jackson mentions a bed of coal which“ was found to have been altered, and was like graphite or plumbago." In Rhode Island the presence of graphite is not adverted to by the state geologist, further than to remark in his analysis, No. 2, of the Portsmouth coal, that it was not plumbaginous. At some new trial openings in the latter neighbourhood, on more than one outcrop, we observed that the mineral appeared to consist almost wholly of graphite. It is remarkably light, spongy, or cellularand is collected and forms an article of sale at a good price, under the name of “British Lustre,” for the usual purposes of plumbago or black lead. Asbestos occurs abundantly, running through the slates which adjoin the coal or graphite bed. Like those of Massachusetts, they are also traversed, and even the coal itself, occasionally, by numerous veins of quartz. All these circumstances combine to satisfy the most skeptical, of the modified or metamorphic character of the coal field of Rhode Island.
* Geological Survey of Massachusetts, p. 162, 1833.