964 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
Seismic world map, 1915—The map of the unstable regions of the 
earth’s outer shell which had been issued by De Montessus in 1906, 
reveals two narrow great circle girdles. (See p. 261, fig. 1.) 
The introduction of instrumental methods with use of the modern 
seismograph has enabled seismologists to extend their studies to the 
floor of the oceans. The seismological committee of the British 
Association published in 1916 a comprehensive world map of great 
earthquakes instrumentally located for a 10-year period (1899- 
1910).18 This map (fig. 2) upon a quite different basis confirmed 
that of De Montessus concerning the twin girdles, but showed that 
these zones extend outward from the margins of the continents into 
the sea and include the trough deeps upon the sea floor. The zone 
of maximum instability, moreover, corresponds to the steep slope 
which joins the mountain arcs of the coasts and islands to the deeps 
which lie parallel to them. The identity of these special earthquake 
girdles with belts of mountain growth—of wrinkled formations 
upon the earth’s surface—seems thus to be confirmed from a new 
quarter. 
Magma pocket below and vent above earth wrinkles —Geologists 
no longer generally believe, as formerly they did, that the earth’s 
interior is molten. ‘The source of the molten rock (magma or lava) 
which issues from active volcanoes is now believed to come from 
reservoirs which are relatively small. There is reason to believe that 
these reservoirs are usually located beneath the arches of the moun- 
tains. Though the earth’s interior is believed to be hot enough to melt 
the rock were it at the surface of the earth and under air pressure 
only, the rock is believed to be kept rigid by the load upon it. The 
formation of the wrinkle at the surface locally lifts this load and thus 
permits a magma reservoir to form beneath it, and above this reser- 
voir the volcanoes naturally develop. 
Omori’s law of geographical succession of earthquakes within 
girdles.—The late Professor Omori, distinguished Japanese seis- 
mologist, has given his adhesion to the view that the earthquakes 
within the great girdles of the earth are connected with mountain 
growth. It was in 1907 in his report upon the California earth- 
quake that he called attention to the geographical order of succes- 
sion of earthquakes within the earthquake girdles. An earthquake 
which occurs within any section of one of the girdles may be regarded 
as relieving the strain by transforming potential into actual energy, 
this relief being partial only beyond the area characterized by heavy 
shocks. The greatest probability of an impending earthquake 
18H. H, Turner and others, 20th Rept. Seismol. Comm., Rept. Brit. Assoc., Manchester 
meeting (1915), 1916, pp. 52-79, pl. 1. 
14 Barth evolution and its facial expression,” 1921, chaps. 3—5. 
