216 The Philippine Journal of Science ms 



observations of the Bertelli microseismoscope or tromometer 

 made in the Manila Observatory. 



With the catalogue of Xavert before us, these observations 

 have been examined again, and we have not been able to find 

 any movement which could coincide with the Nueva Vizcaya 

 earthquakes other than the ones corresponding to the larger 

 shocks mentioned above and three or four other doubtful ones. 

 The experience of seventeen years shows us that this tromometer, 

 still in use at the Observatory, indicates perfectly all earthquakes 

 of any great extent and of intensity III or greater whose 

 epicenter lies within a radius of 200 kilometers from Manila. 



Abella deduced from these facts that the seat of origin of the 

 earthquakes must be very superficial and of small extent, and 

 hence the conclusion that there was little likelihood of any greater 

 ones happening in the future was fully verified. Such earth- 

 quakes could not in any way be classed as tectonic, and hence 

 the author attributed them to volcanic influences, suggesting 

 that the subterranean forces, which at that time had given 

 greater activity to Mayon Volcano, might possibly have extended 

 toward the northwest and affected the Province of Nueva 

 Vizcaya, some 400 kilometers from Mayon. After an examina- 

 tion of Abella's report, it seems evident that the earthquakes 

 of Nueva Vizcaya belong to the rockfall type; and this opinion 

 is strengthened by a consideration of the topography of the 

 province. The whole province is an elevated mountainous region 

 of the nature of a plateau separated from the plains of Luzon 

 to the south by a line of steep cliffs, while on the west it is 

 bounded by a series of peaks whose precipitous western slopes 

 rise abruptly from the deep canon of the Agno River. 



Besides the earthquakes of Nueva Vizcaya, many of those 

 that take place in the Mountain Province, which comprises the 

 former districts of Benguet, Bontoc, and Lepanto, possibly are 

 of the same character. In many parts of this region, particularly 

 in the western side where coraliferous limestone predominates, 

 recent fractures and subsidences are to be met with at every 

 step. The same might be said of other parts of the Archipelago 

 having a similar geological structure. 



DISTRIBUTION OF SEISMIC DISTURBANCES 



In the Philippines as in all other seismic regions of the globe, 

 most earthquakes originate along determined lines which consti- 

 tute special features of the topography of the Archipelago. 



If we add to the Catalogue of violent and destructive earth- 

 quakes in the Philippines 1599-1909, cited above, the earthquakes 



