SLIP DIKES.
The faults of the English coal-seams, therefore, are quite different in form and character from those which are peculiar to our anthracite beds. There the chief or predominating feature of faults in coal is that of trap dikes and " slip dikes," which affect the seams extensively and seriously by "lifting" or changing the horizons of the strata above their proper connections. Thus, we find in the Newcastle district a great many faults of this character, where the seams are disconnected or lifted one above the other, as shown in figure 104, which, however, was not designed to illustrate the English "slip dikes," but rather the local slips in the anthracite fields. It represents, notwithstanding, the English crust-movements or slip dikes,—the result of the trap dikes or their causes.
There is a slight error in the mechanical execution of the above illus tration: the oblique line of the slip should cross the measures or pass down through the bottom in the same manner as the top; and the slate shown under the upper portion of the seam should also be shown under the lower portion: otherwise, this figure represents correctly the English slip dikes. But there those slips, like the trap dikes, affect the measures
throughout; while here the slips, as shown in figure 104, are simply local, and may or may not affect the seams above and below. They are simply moved from their original position by the lateral movements of the crust, instead of the vertical, as in the British coal-fields. The coal in the anthracite fields, when moved by slips, is not materially injured, except by the crushing process, which simply crumbles it. But in the English coal-fields, in the vicinity of the trap dikes, the coal is generally either burned to a cinder or completely coked; in the vicinity of the slip dikes, however, the coal is simply moved from its normal condition or parted in a nearly perpendicular manner.
There are exceptions in the trap-dike faults, however, where the coal is not injured by heat; but in those cases there is evidence of the formation of the dike prior to the formation of the coal, and the coal is deposited on the inclining sides of those dikes at a less angle than where they have been of subsequent occurrence.