The Elements of economic geology

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Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part V. Mineral fuels

Collection:: Economics Books

COAL AND ITS CLASSIFICATION 269 of forests or swamps. Many coal seams rest on a * seat- clay” or underclay, which is a high-grade fireclay as the constituents that act as fluxes in common clay have been extracted by plants that grew on it, when it acted as a soil. The clay often includes tree-roots (Stigmaria) which join trunks that rise through the coal seam, and around them the coal often includes fragments of fossil wood and other plant debris. The sandstone roof may contain fragments of tree-stems and leaves. Above this sandstone may occur another fireclay which is covered by coal; this recurrence may be many times repeated, the fireclay being always under the coal and containing tree-roots. The coal may be inter- rupted in places by bands or * horses” of sandstone; some of them join like the branches of a river, and were probably stream channels that have been filled with sand. Other coal seams, also formed im situ, may rest upon widespread sheets of clay without tree-roots, though the coal may con- tain tree-trunks; these seams grew in situations like the Dismal Swamp of Virginia or fenlands where a level sheet of clay has been covered with swamps and a growth of peat. The theory that coal has been formed from drifted vege- tation is true for other fields. Some coal seams are not underlain by fireclay, but may be interbedded with, or lie beneath it. For example, in the Kilmarnock field in Ayr- shire, the seam known as the Stone Coal is divided by three layers of which two are fireclay. The Hard Seam of Ayr- shire is covered by a bed of fireclay, 3 feet 6 inches thick, which rests on 4 feet 10 inches of coal, including 1 inch of shale, and the seam rests on a soft layer of coaly shale, 2 inches thick. Again in ‘the Patna Coal’ the seam consists in descending order of—fireclay 3 feet, top coal 6 feet, sand- stone 4 feet 9 inches. These fireclays moreover do not con- tain tree-roots, and tree-trunks in the coal may be hori- zontal or inverted, with the roots on the top, indicating that they drifted to their present position. Vertical tree-trunks rising from roots may occur in the Coal Measure sandstones, as well as in coal seams; thus in the fossil grove in White- inch Park, Glasgow, the Lepidodendron trunks are still attached to their roots; but they are in sandstone—not coal. Some coal seams which are shown by their vertical trunks and underlying fireclay to have been formed in situ,

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