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TABLE LVI.—Imports and Exports of Coal, Iron Ore, and Iron to and from the United States, 1880–1881.

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Carboniferous formation has localized the manufacture | variations, refractory beds being interstratified with useof fire-bricks, and by far the larger proportion are pro- less strata largely charged with disseminated carbonate duced in the coal-measure districts. The fire-clays of the coal-measures vary as regards their refractory character, not only in the different coal-fields, but the individual strata in close alternation often present sudden

of iron. The gray color of the coal-measure clays is largely due to the presence of this mineral. The ave rage contraction of the coal-measure fire-clay is a little over 2 per cent.

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The importance of these clays in England may be realized from the fact that in 1880 they produced 3,062,544 tons, valued at £1,635,650. The product consists of fire-clays, potter's clay, china clay, and stone. Prof. J.P. Lesley, State geologist of Pennsylvania, gives his theory of the formation of fire-clay as follows: "The thick and valuable deposit described and repeatedly alluded to in this report as underlying the Kittanning coal is but one of a number of such in other parts of the Appalachian coal fields, equally stimulating to the curiosity of the_geologist. Since Sir W. E. Logan first published the fact that every true coal-bed has its underclay, efforts have been made to explain its cause, but not with entire success. I make the following suggestion: A peat-bog, and still more truly a lake, invaded on all sides by sphagnous vegetation, must have a water-circulation set up by evaporation from the central surface and by percolation from the surrounding land. But this circulation must be feeble, capable of carrying only the finest mud. In the course of ages the amount of clayall of it fine, and all of it of one kind-thus abstracted from the dry grounds and transferred to the peat-filling hollows, must be very large. The dry grounds are chiefly a mixture of fine gravel, rounded grains of quartz, and rounded grains of feldspar. The pebbles and sand are not soluble; the feldspar is. The former remain behind; the latter follows the in-draft, and settles beneath the evaporating surface through and beneath the floating peat. If, however. the area of forming coal-bed be surrounded by clay land, instead of sandy or gravelly land, then the percolation is reduced to a minimum, and with it the solution of feldspar. The supply of water in the pool or marsh is kept up from the land-surface, and not from the underground; and under such conditions the incoming rain-water must be much less muddy and the deposit of under-clay is reduced to a minimum. We might look then for a law somewhat like this: Where a coal-bed has a heavy under-clay, the

margins of its basin were probably sand- or gravelbanks. Where a coal-bed (of such a size as to argue for its great age) has a small under-clay, it grew within a circle of older light clay deposits. Where a limestone underlies a coal-bed, stretches of exposed older limedeposits must have been within drainage-reach all around or on some principal side of it."

"The corollary is, that the thickest under-clays belong to beds next or near the great sand-rock members

of the system; which is a fact: the most persistent fire-clay beds belong near the base of the Lower Productive coal-measures. Another consequence of such a law would be, that sand-rocks lying geologically below great under-clays should be more open and discreet. purer sands and gravels, than others that had not been robbed of so large a quantity of interstitial clay. I say geologically close below, for of course the coal-bed itself

would protect from this operation a sand or gravel deposit lying actually or topically underneath it. When the surrounding land contained iron I would expect ball ore in the fire-clay. In fact, the ponds of New England receive such accumulations."

DEEP COAL-SHAFTS.-The Pottsville shafts of the Philadelphia and Reading Coal and Iron Company,

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