OCEAN BASINS OF THE SECONDARY ERA 243 



the coasts of the areas of seismicity, as may be seen by comparing plates 

 1 to 3 in de Montessus's latest work.* 



As regards the distribution of seaquakes, Eudolph has thus summarized 

 his conclusions for the Atlantic, which has naturally supplied the most 

 ample data: 



"1. Submarine earthquakes and eruptions occur in all depths, in the shal- 

 low as well as the deep sea, on the submarine plateaus, and in the true de- 

 pressed areas. 



"2. The frequency and intensity in the manifestation of the seismic and 

 eruptive forces are not dependent upon the distance from active or extinct 

 volcanoes. 



"3. There are habitual regions of shocks and portions of the sea quite free 

 from them ; with exception of these regions seaquakes occur also isolated and 

 scattered over the ocean." 



There are two most marked zones of seismicity in the Atlantic. One 

 of these extends westward along the parallel from the mouth of the 

 Tagus, thu.s continuing the Mediterranean zone and including Madeira 

 and the Azores. The other is centered on the equator and continues 

 the northern shore of the gulf of Guinea toward cape Saint Eoque. 

 Along the last-mentioned zone in particular the isobaths or submerged 

 contours are sharply offset and the sea-floor presents precipitous slopes. 



Only one wide area has appeared to be immune from seaquakes — the 

 North Atlantic basin and its eastward extension in the trough to the 

 west' of the Azores. It is interesting to note that the northern of the 

 two zones of high seismicity within the Atlantic corresponds to a central 

 band of Milne's oval (H), which is located to the west of the European 

 shore. The other area has, however, not been indicated upon the map 

 of maeroseismic ovals. These studies of Eudolph show that movement 

 is today taking place on the steep slopes bordering the ocean deeps. 



Having in mind the valuable generalization of de Montessus that areas 

 of high seismicity are in correspondence with the geosynclinals, a sug- 

 gestion is here afforded for their extension across the seas, upon which 

 areas no data have hitherto been available. 



Fractures of qnder-sea Cables at Time of Earthquakes 



There is at least one other promising field to exploit for the more defi- 

 nite determination of zones of seismicity within the under-sea regions — 

 the collation of the records of interruptions in telegraphic cables. Milne 

 has already accomplished something in this linef and shown how valuable 



* Les tremblements de terre, Paris, 1906. 



t John Milne : Sub-oceanic changes. The Geographical Journal, London, vol. 10, 

 1897, pp. 129-146, 259-289. 



