Page:Popular Science Monthly Volume 27.djvu/375

Rh more frequent and violent in mountain-regions of recent origin. The Appalachian, which are the oldest mountains in America, are, therefore, very free from earthquakes, because the rocks have, as a rule, long since relieved themselves; while, on the other hand, the comparatively new Rockies are the seat of more frequent tremblings.

There is a theory for the cause of earthquakes, which is rather neglected by scientific men, but which, I think, will explain many of those phenomena not otherwise accounted for.

At the mouth of the Ohio, and at Newburyport, Massachusetts, both of which have in times past been the seat of very severe earthquake shocks, the layers of rocks are not badly bent. And, further, I do not see how the theory of bent rocks can explain the frequent repetition of shocks which we have recently seen in Spain, in Newburyport in 1727, and near the mouth of the Ohio in 1811. These regions had been comparatively quiet for years, and suddenly a severe shock was felt, followed by a series of successive shocks, which, in the case of Spain, have not yet ceased, although the original shock occurred December 24, 1884. The theory, which I have spoken of, and which would explain this, is that there are great quantities of gas accumulated at certain points beneath the surface under great pressure, and that this gas, in its attempt to free itself, bursts open the rocks, causing shock after shock, until it has finally relieved itself sufficiently to remain quiet. This, one will see, is similar to the volcanic theory, except that there is no necessity for the presence of a volcano. What this gas is, does not matter; it may be accumulations of steam, or it may be evolved from petroleum, or it may be carbonic-dioxide gas evolved by acids working on calcareous rocks. In the Newburyport earthquake, which was the most severe ever recorded in New England, large quantities of gas escaped to the surface; and, on the Ohio, gas also escaped. This shock, which was at first wide-spread, finally narrowed itself down to a very limited marshy area and died out.

To sum up, then, it may be said: 1. That in volcanic regions earthquakes are a part of an eruption, premonitory warnings; 2. That in a limestone country the falling in of the walls of caverns may account for some; 3. That in regions of recent mountain-making the sudden release of tension causes many; 4. That the pressure of pen tup gases on the surrounding rocks, which are finally burst, may be the cause of a large number, more especially those which are followed by a long-repeated series of shocks; and, finally, that in any one of these regions either or all of the other causes may (with the exception of volcanic in non-volcanic regions) enter into the production of earthquakes.