The earthquake is a phenomenon connected with volcanic eruptions, and arising from the same great cause; but while the latter are confined to certain mountains, and restricted within narrow limits at the present day, an earthquake is sometimes found to prevail over a very large portion of the earth's surface. To omit the more usual phenomena of earthquakes, we shall speak of but one, which has in some cases been observed, that throws a great light upon the manner in which the stratified rocks have had their levels changed, and been dislocated and distorted in the manner we now find them. We allude to the sudden raising of countries of greater or less extent. Of this we shall quote three several instances from a paper of Arago's.
"During the night of the 28th September 1759, a district of three or four square miles, situated in the Intendency of Valladolid, in Mexico, was raised up, like an inflated bladder. The limits where the elevation ceased may still be determined at the present day, by the fracture of the strata. At these limits the elevation of the ground above its primitive level, or that of the surrounding plain, is no more than thirty-seven feet; but towards the centre of the lifted district, the total elevation is not less than five hundred feet.
"This phenomenon had been preceded by earthquakes that lasted nearly two months; but when the catastrophe occurred, all seemed tranquil; it was announced only by a horrible subterranean noise, that took place at the moment when the ground was lifted. Thousands of little cones, of from six to ten feet in height, called by the natives ovens, arose in every direction; finally six great projections were suddenly formed along a great crevice lying in a north-east and south-west direction, all of which were elevated from 1200 to 1600 feet above the adjacent plains. The greatest of these small mountains has become a true volcano, that of Jorullo, and vomits forth lava.
"It will be seen that the most evident and well characterized volcanic phenomena accompanied the catastrophe of Jorullo; that they were perhaps its cause; but this did not prevent an extensive plain, old and well consolidated, upon which the sugar-cane and indigo were cultivated, from being, in our own days, suddenly raised far above its primitive level. The escape of inflamed matter, the formation of the ovens and of the volcano of Jorullo, far from having contributed to produce this effect, must on the contrary have lessened it; for all these openings must have acted like safety valves, and permitted the elevating cause to have dissipated itself, whether it were a gas or a vapour. If the ground had opposed a greater resistance; if it had not given way in so many points, the plain of Jorullo, instead of becoming a simple hill five hundred feet in height, might have acquired the relief of the neighbouring summits of the Cordilleras.
"The circumstances that attended the formation of a new island near Santorin, in the Greek Archipelago, seem to me also well fitted to prove that subterranean fires not only contribute to elevate mountains by the aid of ejections furnished by the craters of volcanoes, but that they also sometimes lift the already consolidated crust of the globe.
"On the 18th and 22d May 1707, there were slight shocks of an earthquake at Santorin.
"On the 23d, at sun-rise, there was seen between the great and little Rameni (two small islands) an object that was taken for the hull of a shipwrecked vessel. Some sailors proceeded to the spot, and on their return reported, to the great surprise of the whole population, that it was a rock that had risen from the waves. In this spot the sea had formerly a depth of from 400 to 500 feet.
"On the 24th, many persons visited the new island, and collected upon its surface large oysters that had not ceased to adhere to the rock. The island was seen sensibly to increase in size.
"From the 23d May until the 13th or 14th June, the island gradually increased in extent and elevation, without agitation and without noise. On the 13th June it might be about half a mile in circuit, and from 20 to 25 feet in height. Neither flame nor smoke had issued from it.
"From the first appearance of the island, the water near its shores had been troubled; on the 15th June it became almost boiling.