How puny and meagre is the utmost power man can put forth, even by the aid of all his mechanical appliances, when compared with the primal earth forces! Think, or try to think, of the force of pressure that causes the rock-strata to buckle or crumple or bend–layers of rock, thousands of feet thick, made to fold and bend like the leaves of a book–vast mountain-chains flexed and foreshortened, or ruptured and faulted as the bending of one’s body wrinkles or rips one’s clothes. Think of the over-thrusts and the folding and shearing of the earth’s crust. The shrinking of the earth squeezes the rocks to an extent quite beyond our power of conception. “So overpowering has been the horizontal movement in some cases,” says Dana, “that masses of rock thousands of feet in thickness have been buckled up and sheared, or, simply yielding to pressure, have sheared without folding, and been thrust forward for miles along a gently inclined plane. These great reversed faults are termed over-thrusts or thrust-planes. Sometimes such thrust-planes occur singly, at other times the rocks have yielded again and again, great sheets having been sliced off successively, and driven forward one upon the other.” In northern Montana there is an over-thrust of the Cambrian rocks upon the late Cretaceous, of seven or eight miles, carrying with it what is now called “Chief Mountain,” which has been carved out of the extreme end of the over-thrust. The contemplation of such things gives one a sense of power in Nature beyond anything else I know of. The shrinking of the globe as a whole makes its rocky garment too big for it, and this titanic wrinkling and folding results. When the strata snap asunder under the strain, we have earthquakes. During the recent San Francisco earthquake, Mount Tamalpais, across the bay, and all the neighboring heights, were permanently shifted eight or ten feet. The sides of the mountain, it is said, undulated like a curtain. And this shaking and twitching of the great rocky skin of the earth was vastly less, in proportion to the size of the globe, than the twitching and trembling of the skin of a horse when he would shake off the flies, in comparison with the animal’s body.
We see another exhibition of the magnitude of the earth’s forces in what the geologist calls a “laccolite”–a great cave or cistern deep beneath the surface of stratified rock filled with hardened lava. The lava is forced up from an unknown depth under such pressure that, not finding an outlet at the surface, the rock strata, hundreds or thousands of feet thick, are lifted up and arched like so much paper, and in the cavity thus formed the pent-up molten lava finds relief. These lava cisterns or pockets are sometimes uncovered by the process of erosion. The Henry Mountains in Utah are all laccolites. One of them, Mount Hillers, has a volume of about ten cubic miles. Much of the overarching sedimentary strata still covers it. Geologists read the evidence of a similar formation called a “sill” on the west side of the Hudson in New Jersey, forming the Palisades. The lava worked like a giant mole up through and then beneath the Triassic sandstone, lifting the strata up and arching them over a large area. During the millions of years that have elapsed since that time, the layers of superincumbent sandstone have been worn away so that now one sees a wide, smooth, gentle slope of basaltic rock covered by a very thin coat of soil. As one goes by on the train, one sees where the workmen of a stone-crushing plant have cut into the slope and uncovered the junction of the two kinds of rock, one born of water, and one born of fire. The igneous rock sits squarely upon the level sandstone, like a row of upright books standing upon a shelf. I never pass the place but that I want to stop the train and get out and have a close look at the precise spot where this son of Vulcan sat down so heavily and so hot upon his brother of the sedimentary deposits.
Probably no two chapters of the earth’s history differ more than those of the two sides of the Hudson at New York. There is a great break here–a leap from Archaean times on the east side to Mesozoic times on the west. The east side is millions of years the older. Here is the original Plutonic or Azoic rock which apparently has never been under the sea since it was first thrust up out of the fiery depths. The west shore, including the Palisades, belongs to a much later geologic era. The original granite here is buried under vast deposits of sedimentary rock of the Triassic age–the age of the giant reptiles, the remains of one of which has recently been found embedded in this sandstone, near the river’s edge. As the traveler’s eye follows along the even, almost level line of this escarpment of the Palisades, let it re-create for him the strata of the old Triassic sandstone that were millions of years ago piled high upon it,–how high can only be conjectured,–but which have been removed grain by grain under the eroding power of the forces of air and water that now seem to caress the huge wall so gently. Ah! geologic Time, what can it not do? what has it not done? The old sill of Vulcan now presents a nearly vertical front to the Hudson, forming the Palisades, showing that some leaves of the earth’s history here are missing, buried probably beneath the waters of the river. There is evidently a line of fault here, and the west side has been lifted up out of the old Mesozoic seas, probably in the convulsions that poured out the lava of the trap rock.