TRAP DIKES.
Figure 103 represents the general form and effects of the English trap dike faults, which consist of lava ejected or forced from the molten bowels of the earth through its confining crust by the forces of contraction.
The cooling and contracting process which the rocky crust of the earth has undergone, or is undergoing, on its molten and uncontracting core, naturally produces that irresistible effOrt to escape which we see exemplified in dikes, volcanoes, &c. In England, where no great volcanic peaks exist, and where the formations are uniform, and, consequently, without those peculiar weak points which yield to the forces of contraction, the condensed lava bursts through the confining crust in long lines of trap, which are known as trap dikes. In the English coal measures the dikes thus formed by the lava are frequent. They occupy long parallel lines across the coal measures and through the adjoining country,—often from sea to sea. These trap dikes burst through the strata nearly at right angles, but some times lean with the line of "cleavage." The coal-seams are cut and divided by the lava, and frequently one portion is carried upwards by the lifting power of the molten mass until the connecting points are several hundred feet "out of place," as in the instance of the "90-fathom dike" of the Newcastle coal-field.
Thus the forces of contraction are exerted according to the geological character of the country or district in which they occur. In Mexico and other volcanic countries the lava escapes through volcanic vents, as the earth's crust contracts until its rocky bands are forced to yield to that power which nothing can resist. In Chapter IV. we stated an instance of the terrible force with which volcanic lava is vented. It is said that Cotopaxi, which is nearly 19,000 feet high, has projected lava 6000 feet above its summit, and that it once threw a stone 109 cubic yards in volume to a distance of nine miles.
In the United States we find the forces of contraction east of the Rocky Mountains exerted on the foliated and yielding strata of the Atlantic slopes ; while west of the Rocky Mountains we find it vented in volcanic eruptions, and in England we find the trap dikes. We allude to the exertion of those forces, in these instances, however, since the formation of coal: prior to that period, volcanic eruptions were general in all sections.