The Student’s Elements of Geology

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Author: Charles Lyell

Blending of Coal-Seams.

Both in England and North America seams of coal are occasionally observed to be parted from each other by layers of clay and sand, and, after they have been persistent for miles, to come together and blend in one single bed, which is then found to be equal in the aggregate to the thickness of the several seams. I was shown by Mr. H.D. Rogers a remarkable example of this in Pennsylvania. In the Shark Mountain, near Pottsville, in that State, there are thirteen seams of anthracite coal, some of them more than six feet thick, separated by beds of white quartzose grit and a conglomerate of quartz pebbles, often of the size of a hen’s egg. Between Pottsville and the Lehigh Summit Mine, seven of these seams of coal, at first widely separated, are, in the course of several miles, brought nearer and nearer together by the gradual thinning out of the intervening coarse-grained strata and their accompanying shales, until at length they successively unite and form one mass of coal between forty and fifty feet thick, very pure on the whole, though with a few thin partings of clay. This mass of coal I saw quarried in the open air at Mauch Chunk, on the Bear Mountain. The origin of such a vast thickness of vegetable remains, so unmixed, on the whole, with earthy ingredients, can be accounted for in no other way than by the growth, during thousands of years, of trees and ferns in the manner of peat— a theory which the presence of the Stigmaria in situ under each of the seven layers of anthracite fully bears out. The rival hypothesis, of the drifting of plants into a sea or estuary, leaves the non-intermixture of sediment, or of clay, sand, and pebbles, with the pure coal wholly unexplained.

(FIGURE 430. Uniting of distinct coal-seams.)

The late Mr. Bowman was the first who gave a satisfactory explanation of the manner in which distinct coal-seams, after maintaining their independence for miles, may at length unite, and then persist throughout another wide area with a thickness equal to that which the separate seams had previously maintained.

Let A-C (Figure 430) be a three-foot seam of coal originally laid down as a mass of vegetable matter on the level area of an extensive swamp, having an underclay, f-g, through which the Stigmariae or roots of the trees penetrate as usual. One portion, B-C, of this seam of coal is now inclined; the area of the swamp having subsided as much as 25 feet at E-C, and become for a time submerged under salt, fresh, or brackish water. Some of the trees of the original forest A-B-C fell down, others continued to stand erect in the new lagoon, their stumps and part of their trunks becoming gradually enveloped in layers of sand and mud, which at length filled up the new piece of water C-E.

When this lagoon has been entirely silted up and converted into land, the forest-covered surface A-B will extend once more over the whole area A-B-E, and a second mass of vegetable matter, D-E, forming three feet more of coal, will accumulate. We then find in the region E-C two seams of coals, each three feet thick, with their respective under-clays, with erect buried trees based upon the surface of the lower coal, the two seams being separated by 25 feet of intervening shale and sandstone. Whereas in the region A-B, where the growth of the forest has never been interrupted by submergence, there will simply be one seam, two yards thick, corresponding to the united thickness of the beds B-E and B-C. It may be objected that the uninterrupted growth of plants during the interval of time required for the filling up of the lagoon will have caused the vegetable matter in the region D-A-B to be thicker than the two distinct seams E and C, and no doubt there would actually be a slight excess representing one or more generation of trees and plants forming the undergrowth; but this excess of vegetable matter, when compressed into coal, would be so insignificant in thickness that the miner might still affirm that the seam D-A throughout the area D-A-B was equal to the two seams C and E.

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Chicago: Charles Lyell, "Blending of Coal-Seams.," The Student’s Elements of Geology, ed. Bryant Conant, James and trans. Babington, B. G. (Benjamin Guy), 1794-1866 in The Student’s Elements of Geology Original Sources, accessed September 19, 2018, http://www.originalsources.com/Document.aspx?DocID=DCCIZD1USET24FF.

MLA: Lyell, Charles. "Blending of Coal-Seams." The Student’s Elements of Geology, edited by Bryant Conant, James, and translated by Babington, B. G. (Benjamin Guy), 1794-1866, in The Student’s Elements of Geology, Original Sources. 19 Sep. 2018. www.originalsources.com/Document.aspx?DocID=DCCIZD1USET24FF.

Harvard: Lyell, C, 'Blending of Coal-Seams.' in The Student’s Elements of Geology, ed. and trans. . cited in , The Student’s Elements of Geology. Original Sources, retrieved 19 September 2018, from http://www.originalsources.com/Document.aspx?DocID=DCCIZD1USET24FF.