1909.] OUTLOOK OF SEISMIC GEOLOGY. 285 



are distributed upon the flanks of the mountain, we find that the 

 majority of these are not radial to the mass at all, but comprise a 

 network. A notable instance of a line of craters not in radial rela- 

 tion to the central cone is furnished by the chain of Monti Segreta, 

 Nocella, Pizzuta, Gervasi, Arso and Difeso. Nearly parallel to this 

 chain is that of the Monti Mazzo, S. Leo, Rinazzi, Guardiola and 

 Albano. A map of these and other monticules upon the flanks of 

 Etna has been already published by the writer.^^ It is, therefore, 

 not only possible, but extremely probable, that in many instances the 

 earthquakes which so generally accompany the rending of a volcanic 

 cone, are directly associated with the opening of, and perhaps a 

 differential movement upon, those fractures in the basement of the 

 mountain which are a part of the larger fracture system of the 

 district. Lacroix has recently shown that a nctzvork of fissures 

 appeared upon Etna in connection with the eruption of 1908."^^ 



The Conditions of Earth Strain During the Grozvth of Block 

 Mountains. — If we consider any circumscribed portion of the earth's 

 crust within which mountains are growing through the adjustment 

 by individual blocks or compartments of the crust, it is necessary 

 to assume that the superficies is increased during the process. Indi- 

 vidual blocks may indeed be actually depressed as a consequence of 

 the adjustment, but yet the average movement must be assumed to 

 be upward rather than downward. Such a conclusion is, however, 

 in contradiction of the generally accepted view that mountain growth 

 comes about through a reduction of superficial area from secular 

 cooling. This very obvious difficulty in the way of adopting the 

 Schollen conception of mountain structure has been quite generally 

 recognized, and we have already seen how Oldham, in seeking the 

 cause of the great Assam earthquake, was led to reject the theory, 

 even though the vertical faults and the differential changes in level 

 were plainly to be observed. 



In the opinion of the writer, the recent study of " distant " earth- 

 quakes by modern seismographs has removed this difficulty in the 

 way of a general acceptance of the fault-block theory. By extend- 



^ Gerland's Bcitracge s. Gcophysik, Vol. 8, 1907, pp. 348-350, PI. 10. 

 ^''" L'eruption de I'Etna en avril-mai 1908, Revue generale des Sciences 

 pures et appliquees. 2.0" annee, 1909, pp. 298-314. 



