Page:Popular Science Monthly Volume 86.djvu/455

Rh Lisbon earthquake was also in a non-volcanic area. Likewise, the region of occurrence of the recent Italian earthquake shows that it also is situated in a non-volcanic area. Furthermore, as a general rule—though earthquakes often happen in volcanic regions due to explosive shocks—the most disastrous and most powerful earthquakes occur in non-volcanic areas and are tectonic in their nature. They—the latter—are due to a series of rapid shocks which accompany movements along lines of weakness, such as faults or previously made fissures due to mountain-making movements. The throw of these faults need not be very great in order to produce disastrous results.

The frequent coincidence in the distribution of earthquakes and volcanoes may be said to be due to their dependence upon a common cause, in the sense that volcanoes may be regarded as incidental to and the result of great earth movements. Such movements evidence either the collapse of large areas of the earth's crust, or crustal uplift—each a warping, and both due to movements of an orogenic or mountain-making character. These crustal crinklings, moreover, which occur on a vast scale, are not confined to one period of the earth's history, but have been experienced by the earth at different times since the very beginning and are likely to continue to the end—which, considering the physics of the earth, and various well-known geological facts, appears to be fully as remote as the genesis of the earth itself.