Page:Popular Science Monthly Volume 86.djvu/454

450 As is well known, the islands of the former are almost constantly shaken by earthquakes. This region and the East Indian region, as will be observed from the map, are not sharply marked off from one another.

Along the western coast of North America, there are three earthquake regions. These are shown separately, but as a matter of fact are connected by narrow strips, marking areas of more restricted seismic disturbance. The central belt is thus to be connected, on the one hand, with the southern belt, and, on the other, with the northern belt. The most northern of these three belts lies along the coast of Alaska and British Columbia; the central or California-Ecuador belt, begins in California, includes all of Central America, and ends in Ecuador; the southern, or Peru-Patagonia belt, follows the coast of South America from Peru to Patagonia.

Both the West Indian and the South Indian-Madagascar earthquake regions extend over areas which have experienced more or less profound downward crustal warping. The West Indian region, possessing definite features of marked structural instability, suggest the Malaysian area, which latter, however, is characterized by young and growing mountains, but also exhibits in certain of its parts warping of the kind previously alluded to. The West Indian area is also a region of unusually active earthquakes which are due doubtless to a continuance of the earth's movements by which the old continent of Antilla has been broken up into the islands that go to make the present Archipelago. The South Indian-Madagascar region is supposed to mark the site of a former land mass now vanished under the Indian Ocean. This earthquake region extends from South India in a southwesterly direction to the island of Mauritius and to the east of Madagascar.

The North Atlantic may be divided into three earthquake regions—of these three, one lies northeast of Iceland and parallels the coast of Scandinavia. The second and largest of these three regions extends from North Africa in a northward direction along and past the coasts of Spain and Portugal to the west of the Bay of Biscay, and thence to the west of Ireland. It was along the line of movement here described that there developed the terrible Lisbon earthquake. The third of these areas, which is about the size of the first and the least active of the three, parallels the eastern coast of the United States and includes the islands of the Bermudas.

Some of these earthquake regions, such as the last of the eleven mentioned, are notably free from violent volcanic activity; but, as we have seen, even in a region containing an active volcano the most powerful earthquakes often affect the non-volcanic districts. Thus, to recapitulate, the appalling Calabrian earthquake of 1783, though near the volcanic areas of Sicily and not a great distance from Vesuvius, affected a part of southern Italy where there are no volcanic rocks. The