DYNAMIC GEOLOGY
Treats of the forces that move things on or beneath the earth’s surface. The Drift shows not a little confusion. Things are evidently in an abnormal condition, and strangely mixed. Some of the disturbing causes are obvious. Currents of the atmosphere and ocean have done much, but are not sufficient to account for all the phenomena. Boulders brought from ledges north of the great western lakes, are found scattered over all the western states, some much battered on the passage, others bearing only marks of long exposure to the elements. Deep furrows have been plowed in the rocks and hill tops over which they passed, at an elevation of thousands of feet above the level of the sea. Currents of water could never have lifted such huge masses from the lower to higher levels, or transported them any such distances. Icebergs or glaciers have evidently moved over the whole Drift region with fragments of rocks and pebbles frozen into their lower surface, that, like huge rasps, both cut away and polished the hardest rocks, at the same time bearing forward the boulders and whatever else chanced to be held in their cold embrace. There are other footprints of many and very great changes that have been wrought. Though many persons have erroneous impressions of the inequalities on the earth’s surface, the height of the loftiest mountains being but little when compared with the earth’s diameter, yet there is evidence that the normal condition has not been preserved. Large districts have, even within the historic period, been lifted far above their former level, and others sunk as much below. New islands have appeared in the midst of the sea, while others have sunk out of sight. Multitudes now live on what was once the bed of the sea, “in which were things innumerable, great and small beasts;” and ships sail over territory once covered with the habitations of living men. Rocks of immense thickness have been broken and the parts lifted into a vertical position, and many such great changes have taken place. What wrought them? It is safe to say that at least two forces have been operating, the one more gradual than the other. The cooling of the internal mass must cause contraction, which, in a globe of such dimensions, would be sufficient to break the strongest rocks constituting its shell. This force, when properly directed, might lift the rocks, and even throw them back on other strata of more recent formation. Then the expansive force of the gases within, when raised to their highest tension, is enough to cause earthquakes, and pour through the partially opened craters, or where the barriers are made less secure, floods of lava that are in time changed into rocks of that peculiar class. The vent will be found where the crust above the struggling giant is weakest, whether that be on the mountain top where the rocks had been shoved up into a vertical position, or at the bottom of the sea.
The dynamics of geology suggest problems of no ordinary interest, but our narrow limits forbid even a statement of them.