THIS IS THE HAND OF VULCAN, TOO
Were you ever down by the seashore in a storm? If so you remember how the ground under your feet shook when a great wave rushed into some narrow passage or crevice in the rocks, and was tossed high in the air in spray. Then just imagine molten lava, which is many times heavier than water, driven into a crack in a rock with the force of a cannon-ball. That's how it happened. That's how those dark strokes in the rock with their heavy shading were made.
This was done in the depths of the earth; not on the surface where you see these rocks now. They used to have piles of other rocks above them, but these in course of time have been weathered away. This is known, not only from the marks of the wearing but from the fact that these dikes, as well as the rock into which they have been driven, are crystallized, wholly or in part. Such crystallizing, as we know, takes place away down in the earth.
Dikes are very common. In some places you find the rocks fairly laced with them. The picture of the dikes in the granite shores at Marblehead also shows (in the horizontal plan) many "faults" or slips of the rock since the dike was made, and each slip probably gave rise to an earthquake. So you see there's the story of a terrible time written on those quiet old residents by the sea.
THE GIANT'S CAUSEWAY
Here is a still more striking example of the formation of columns in lava—the Giant's Causeway. Here are 40,000 columns, packed like the cells of a honeycomb, and they slope to the pavement in the foreground that gives the mass its name. That bees should make their little honey-jars in such regular form is wonderful enough, but think of lava shaping its own self into columns like that!
DID MR. VULCAN USE A STEAM PILE-DRIVER?
Just what power Mr. Vulcan used to drive the dikes is not known for sure, but I'll tell you how it is supposed to have been done. Remember that all rocks that are deep down in the earth contain water, shut up in their pores. Then remember how hot it is down there and how this heat would make steam right in the rocks. Then let the rock above be cracked by the movements of the earth crust, and this crack extend down to where these hot rocks are, the pressure, being released along that crack, the melted rock (lava) would rush up, as it does in connection with the eruptions of volcanoes, and the exploding steam would help drive it.