Page:Popular Science Monthly Volume 31.djvu/380

366 through, the Aleutian Islands to Kamchatka, and thence southward through the Japanese Islands, the Philippines, and the Moluccas, to Sumatra and Java. Another branch seems to run from Sumatra, through New Guinea, to New Zealand, and the closed curve may perhaps be completed through the Antarctic regions, which are known to be volcanic. Returning to the first branch which we traced as far as Java, to the westward the seismic areas become more patchy and less linear. It may, however, perhaps be maintained that the ribbon runs on through India, Persia, Syria, the Eastern Mediterranean, Greece, and Italy.

This grouping of seismic areas into a ribbon does not comprise all the regions of earthquakes, but it must rather be taken as meaning that there is one great principal line of cracking of the earth's surface. In speaking here of earthquakes, those sensible shocks are meant which are sufficiently severe to damage buildings, for, as will be explained below, there is reason to believe that the whole earth is in a continual state of tremor.

Seismic areas are not absolutely constant in their limits, and cases are known where regions previously quiescent have become disturbed. It seems likely that the recent disastrous earthquake at Charleston belongs to the West Indian system of seismic activity, but there is no reason to suspect a permanent extension of the West Indian area so as to embrace the Southern States. On the contrary, it is far more probable that this disastrous shock will remain a unique occurrence. The previous experience of great earthquakes, such as that of Lisbon in the middle of the last century, shows, however, that the inhabitants of Charleston must for the next year or two expect the recurrence of slight shocks, and that the subterranean forces will then lull themselves to sleep again.

With regard to the distribution of earthquakes in time there is no evidence of either decrease or increase within historical periods, and although physical considerations would lead us to suppose that they were more frequent in early geological times, geology at least can furnish no proof that this has been the case.

A great deal has been written on the causes of earthquakes, and many of the suggested theories seem fanciful in the highest degree. It is clear, however, that the primary cause resides in the upper layers of the earth, and that the motive power is either directly or indirectly the internal heat of the earth. The high temperature of the rocks, in those little scratches in the earth's surface which we call mines, proves the existence of abundant energy for the production of any amount of disturbance of the upper layers. It only remains to consider how that energy can be brought to bear. One way is by the slow shrinking of the earth, consequent on its slow cooling. Now, the heterogeneity of the upper layers makes it impossible that the shrinkage shall occur