The ARCHED ROCK is an isolated mass of self-sustaining rock, on the eastern facade of cliffs; it offers one of those coincidences of geological degradation in which the firmer texture of the silicious and calcareous portions of it have, thus far, resisted decomposition. Its explanation, is, however, simple: The apex of this geological monument is on a level, or nearly so, with the Fort Holmes summit. While the diluvial action, of which the whole island gives striking proofs, carried away the rest of the reticulated or magnesian limestone, this singular point, having a firmer texture, resisted its power, and remains to tell the visitor who gazes at it, that waters have once held dominion over the highest part of the island.
Before dismissing the subject of the geological phenomena of this island, it may be observed that it is covered with the erratic block or drift stratum. Primitive or crystalline pebbles and boulders are found, but not plentifully, on the surface. They are observed, however, on the highest summit, and upon the lower plain; one of the best localities of these boulders, exists on the depressed ground, leading north, in the approach to Dousman's Farm, where there is a remarkable accumulation of blocks of granite and hornblende drift boulders. The principal drift of the island consists of smooth, small, calcareous pebbles, and, at deeper positions, angular fragments of limestone. Sandstone boulders are not rare. Over the plain leading from the fort north by way of the Skull Rock, are spread extensive beds of finely comminuted calcareous gravel, the particles of which often not exceeding the size of a buck-shot, which makes one of the most solid and compact natural macadamized roads of which it is possible to conceive. Carriage wheels on it run as smoothly, but far more solid, than they could over a plank floor. This formation appears to be the diluvial residuum or ultimate wash, which arranged itself agreeably to the laws of its own gravitation, on the recession of the watery element, to which its comminution is clearly due. It would be worth transportation, in boxes, for gravelling ornamental garden-walks. The soil of the island is highly charged with the calcareous element, and, however barren in appearance, is favorable to vegetation. Potatoes have been known to be raised in pure beds of small limestone pebbles, where the seed potatoes had been merely covered in a slight way, to shield them from the sun, until they had taken root.