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.