THE RICHMOND, OR TIDE-WATER, COAL-FIELD.
This coal-field crosses the James River about 12 miles west of Rich mond, Virginia, and extends in a north-and south direction to the Appo mattox, 20 miles west of Petersburg. It is about 30 miles long by 5 miles wide, and contains an area of about 150 square miles. But perhaps less than half of this area contains available coal, owing to the undulations of the granite, which rises to the surface repeatedly in many sharp and abrupt peaks.
The basin is irregular, and is surrounded by a fine-grained granite, resembling sienite, which produces an excellent building-material, almost equal to marble in appearance. The interior of the basin presents peaks of granite of various textures, with occasional dikes of trap and porphy ries. The field consists of a series of deep basins, the whole resembling the vast crater of some expired volcano, studded with sharp peaks and surrounded by rugged and irregular sides. The inequalities of the basin are in a measure modified by the sedimentary deposits which preceded the coal. But these deposits only filled the deeper hollows, leaving the coal in many cases to be stratified on a granite base. It seems evident that no subsequent crust-movements of importance have taken place in those Eastern and late formations. The deposits are thickest in the deeper basins or synclinals, limited on the inclining sides, and very thin on the anticlinals, thus proving positively that the basins existed much in their present condition when these deposits took place.
In the deep and inverted basins of the anthracite regions of Pennsyl vania this is not the case; for the strata are frequently thicker on the inverted side than in the bottom of the deep basins or the more uniform dips, as may be observed at Pottsville, where the conglomerate is thicker— though leaning in an inverted manner from the perpendicular—than it is in the bottom of the basins.
We find some comparatively steep dips in the Richmond coal-field, con sidering them as original formations; but it is rare to find any available coal on these abrupt dips. The coal, as well as the sedimentary strata, is
always thickest in the depressions, or synclinals, and thinnest on the saddles, or anticlinals. There are no slips and "heaves," as represented in Taylor's Statistics. All the irregularities are caused, with one or two rare excep tions,—to be subsequently described,—by the original inequalities of the granite floor, as approximately illustrated in figure 133.
The inequalities are much greater locally than the generalized section portrays, and more distinctly represented by figure 107, under the chap ter on Faults, &c. Those "troubles," as the cone-like synclinals are locally named, are numerous and really troublesome. The basins vary in depth ; but the principal ones are about one thousand feet deep. The dips of the measures on the east sides are from 20° to 40°, and sometimes much greater; but on the west the dips (dipping east) are from 25° to 80°, or sometimes perpendicular; and generally the descent to the basins on the east-dipping strata is in steps, the coal resting invariably on the less-inclining strata at the foot of each abrupt descent. The basins generally contain large deposits of coal, varying from 20 to 60 feet in thickness, particularly on the South Side, where all the available coal is found in one bed near the base of the measures, and not much above the granite.
Figure 134 represents the measures in the basins on the south side of the anticlinal, near James River. This anti clinal in a manner divides the northern end of the coal-field from the centre and south. It rises about two miles south of the river, and between it and the Black Heath and Mid lothian mines, as marked on the map. The measures to the south of this anticlinal are as shown in this figure, and are materially different from those on the north side of the anti clinal, as will be noticed farther on.