. Elements of geology : a text-book for colleges and for the general reader. Geology. Fig. 105.—Shocks in Mines would reach, the surface by refraction, but a portion would be reflected and return into a b, concurring and interfering with the advancing waves, and producing great commotion in these strata. 5. Shocks less severe in Mines.—This case is probably more common than the last. It was notably the case in the earthquake of 1872 in Inyo County, Califor- nia. While the sur- face was severely shaken, many houses destroyed, and large fissures formed in the earth, the miners, sev- eral hundred
. Elements of geology : a text-book for colleges and for the general reader. Geology. Fig. 105.—Shocks in Mines would reach, the surface by refraction, but a portion would be reflected and return into a b, concurring and interfering with the advancing waves, and producing great commotion in these strata. 5. Shocks less severe in Mines.—This case is probably more common than the last. It was notably the case in the earthquake of 1872 in Inyo County, Califor- nia. While the sur- face was severely shaken, many houses destroyed, and large fissures formed in the earth, the miners, sev- eral hundred feet be- low the surface in the hard rock, scarcely felt it at all. This is probably, at least partly, explained as follows : As long as the wave travels within the earth, motion of the particles is restrained by the work of elastic compression; but, as soon as the sur- face is reached, the motion becomes free, and the velocity of shock is far greater than before, often so great as to throw bodies high in the air. The phenomenon is exactly like that in the familiar experiment of the ivory balls: when the first in the series is struck, an elastic wave of compression passes through all, but only the last one moves. 6. Bridges.—In a manner somewhat similar are to be accounted for the phenomena of bridges. In the earthquake regions of South America there are certain favored spots, often of small extent, which are partially exempt from the shocks which infest the surrounding coun- try. The earth- quake-wave seems to pass under them as under a bridge, to reappear again on the other side. The mere inspection of Fig. 106 will explain the probable cause of this exemption, viz. : reflection from the under surface of an isolated mass of soft, inelastic strata, c c. 7. Fissures.—The ground-fissures, so commonly produced by earth- quakes, are sometimes of the nature of the great fissures of the crust, which are the probable cause of earthquakes. Such great fissures are usually whol
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