THE WHIN SILL.
Another singular formation or feature of the Great Northern coal-field is the "whim sill," marked g in the section, which exists between the upper and lower Carboniferous limestones, and in the vicinity of the lower coal measures. This is a trap, or basaltic rock, spread over and between the limestones, and seems to be coextensive with the field. It evidently origin ated from some volcanic eruption of the time, and may have resulted cotemporaneously with some of the great dikes.
It is extremely doubtful that those dikes are the invariable formations of a period subsequent to the coal era, as many suppose, since the coal is only occasionally affected by the heat of those igneous rocks, as it would be were they injected through it. The coal has, in a few instances, been found charred near the dikes; but generally they simply terminate in their vicinity, as might be expected in the deposition of the coal-beds if subse quently formed.
We find cases of veins or seams being consumed to a cinder, and others charred to a perfect coke, by dikes of subsequent formation, which have been injected through or between the coals. The Tuckahoe portion of the Richmond coal-field, in Virginia, lying principally on the north side of the James River, is a case in point. Here we find a "whin sill" precisely similar to that of the Newcastle field in England, injected between two coal-seams, as may be seen in the sections illustrative of the Richmond field. The trap found a vent beneath the upper seams of coal, which it burned to a complete cinder, while the succeeding seam below was formed into a beautiful and useful natural coke, which is in great demand as a household fuel.
The lower coal measures of the English formations are fully developed in the Great Northern coal-field. The seams of coal in this lower series crop out in the mountain limestone, or between the upper and the lower Carboniferous limestones.
The upper beds are two in number, and have been found of workable thickness; the lower beds are generally three in number, though they fre quently develop more numerously. The "whin sill" is between the two series of seams, but far enough from either to prevent a coking or char ring influence.
Those seams form a valuable body of coal at Scremerston, in the vicinity of Holy Island, some 30 miles from the extremity of the Northern coal field, on the Coquet. Here this lower coal has developed to a thickness of 90 feet; and it is generally believed that the coal of the Lothians, in Scotland, still farther north, is of the same formation. Another basin of the lower coal series is found at Plasketts, to the west; while still west of that are the coals of Canobie, supposed to belong to the true coal measures.
The dip of the measures, generally, is from east to west in the Northern coal-field. They rise from beneath the sea towards the west at the rate of one in forty, or something over 100 feet to the mile. In the deepest parts of the basin the coal appears to be thinner than towards the outcrops, which seems to be the rule all the world over, viz.: in all very deep basins. The magnesian limestone lies over the deepest portions; but we cannot suppose that had any effect on the coal, which must have been formed long before the limestone.