Page:Popular Science Monthly Volume 66.djvu/408

404 1,200, 1,500 or even in places to 2,000 fathoms. These depths are reached at from two to five miles beyond the hundred fathom line.

Rising from the floor of the sea some ten or twelve miles to the southwest of this oval plateau is another much smaller one, which would still be some 150 or 200 feet under the surface were the sea to recede, as we have imagined, thirty feet. This is known as the Challenger Bank.

Ten miles beyond this is another similar plateau of about the same size and height, called the Argus Bank. These two banks are in reality a part—two detached peaks, as it were—of the great submarine mountain of which the Bermuda Islands are the visible summit. For

while the floor of the ocean sinks within five miles to about 1,500 fathoms, these plateaus are separated from each other and from the Bermuda plateau by less than half that depth.

The present land area of all the Bermuda Islands is composed of calcareous rock which varies from a loose sand to a firm, hard, semi-crystalline limestone that resounds to the blow of the hammer. What underlies this, no one yet knows. The deepest excavations—those for the new Navy Yard docks at Ireland Island—have not disclosed any other kind of rock. The numerous deep cuts for roads (Fig. 7) and the quarries which are met with in all parts of the islands tell the same story. The rocks are composed of wind-blown calcareous sand. This sand, contrary to what was formerly supposed, is not composed of broken down corals. These are present only in small proportion, the