Page:The American Cyclopædia (1879) Volume IV.djvu/756

740 this great rock, which is a quartzite, and often a conglomerate rock, 80 ft. thick in the anthracite measures. It is sometimes accompanied by a stratum of white quartz secretions, F, or Holmes or conglomerate, even in the western portions of the field, which are often mistaken for water-worn pebbles. This is a great landmark in the Appalachian coal fields, which cannot well be mistaken, and yet it is often misplaced. Above this exists the group F, which consists of two thin impure beds, divided by a few inches of fire clay, known as the rough bed in the anthracite fields, where it is 5 to 7 ft. thick, and as a single bed in the Allegheny field, 1 to 2 ft. thick of slaty and sometimes 3 ft. of cannel coal. It seems to be a true horizon of coal, but is seldom found in merchantable quantity or quality. Above these are from 200 to 300 ft. of shales, slates, sandstones, and limestones, followed by the bed G, which is the large and celebrated Pittsburgh bed, remarkable for its production of excellent gas, coking, steam, and household coal, combining all the qualities of every variety of bituminous coal except the block and cannel. It ranges from 6 to 12 ft. in thickness, averaging from G, or Primrose. 6 to 8. Between these great beds, E and G, exist from 300 to 450 ft. of unproductive strata, which contain no workable beds of coal. These are known in Pennsylvanian nomenclature as the lower barren measures, which are as distinctly marked in the anthracite as in the bituminous fields of this state. It may be briefly stated that all the coal beds and coal measures existing in the anthracite fields above G are found in some portions of the Alleghany field; but the coal beds are thin, rarely workable, and cannot be identified. From 1,000 to 2,000 ft. of coal measures are supposed to exist above G; but these are known as the upper barren measures, and are made up chiefly of shales, with a few coarse sandstones and massive limestones, one of which is 70 ft. in thickness, and is distinctly defined over a large area. The general average thickness of the coal measures between B and G is 1,000 ft., but varies from 500 to 1,200 ft. From the carboniferous limestone to B, including the groups O and A, the thickness of the strata is from 200 to 500 ft., and the total thickness of the coal measures about 3,000 ft. in Pennsylvania, with a minimum thickness of 30 ft. and a maximum of 50 ft. of coal.—The distribution of the deposits of coal in North America, is well adapted for the supply of the wants of the present inhabitants. The largest population is along the Atlantic coast, and the best coal, that of the anthracite fields of Pennsylvania, happens to be situated nearer the largest markets than any other, being less than 200 m. from New York and less than 100 m. from Philadelphia. The basins producing it are small, containing in all but 470 sq. m.; but the beds are very large and numerous, and the quantity produced is about half of all the coal mined in the United States. (See, , and .) In the eastern central part of Pennsylvania, where the anthracite basins are situated, great disturbances of the strata have taken place after they were deposited, caused by the gradual upheaval or subsidence of alternate portions in N. E. and S. W. lines, so as to throw them into a waving form. This disturbance was greatest toward the S. E., and the rock arches become wider and flatter as we go N. W.; but they extend S. W. entirely across this state and Maryland, and their effects are even seen in the coal field of eastern Ohio. All anthracite coal is found in regions where the strata have been considerably disturbed, or where from local causes it has been subjected to heat. Next westward from the anthracite in Pennsylvania the coal is semi-bituminous, and still further west it is of the ordinary bituminous character, the quantity of volatile matter constantly increasing toward the central part of the field. The carboniferous formation terminates in the northern part of Pennsylvania, and the division into counties of that district happens to correspond with six of the great flexures of the strata before mentioned, which give rise to six coal basins. Some of these from their far northern position contain some of the richest and most productive mines in the state. They produce, for the supply of the coalless country north of them, the variety commonly called Blossburg, which is used for steam and manufacturing purposes. The deposits of coal extend in this northern district along the middle or bottom of the basins only, in lines of small detached fields or chains of basins, which are more extensive as they are followed S. W. until they become uninterrupted prongs or finger points. Still further S. W. in Pennsylvania the lower beds arch over portions of the intermediate anticlinals, and in the S. W. part of the state, in the Pittsburgh country, the four or five lower beds which alone occur further N. disappear on the surface, dipping under a red and gray shale formation in which are no coal seams. Above these barren measures in the highest ground about Pittsburgh appears another bed of excellent coal, named after that city, from which all the coal is mined that is used in the S. W. part of the state, large quantities of it being also sent down the Ohio and Mississippi rivers. Pennsylvania not only supplies the United States with all the popular fuel anthracite, but she also produces more bituminous coal than any other state, of which she