212 The Philippine Journal of Science mz 



(/) Recent alluvium from the mountains deposited upon coral 

 shelves results in a greater or less development of coastal plains 

 around all of the islands. The coastal plains are for the most 

 part negligible, but some of the intermontane plains are very im- 

 portant. The northern three-quarters of the central plain of 

 Luzon is largely alluvium. The Albay Plain is largely alluvium 

 as is the case in the great valleys of the Cagayan, Agusan, and 

 Cotabato Rivers. Also, the central plain of Panay shows a 

 very considerable accumulation of detrital material. 



Around Manila we have, in addition, a great area of pyro- 

 clastic material which is cut through by the Pasig River. This 

 is known from well logs and river sections to be at least 100 

 meters thick. 



TYPES OF SEISMIC DISTURBANCES 



Modern seismology, in rejecting every agent and force ex- 

 terior to our planet as causes of earthquakes, reduces the classes 

 of shocks to the following three types: volcanic, tectonic, and 

 rockfall. 



Many examples of these three types are to be found in the 

 seismological records of the Philippine Weather Bureau, and a 

 few of the more characteristic will be briefly mentioned. 



To the volcanic type belong all those earthquakes that are 

 intimately connected with volcanic eruptions caused by explosions 

 or sudden outbursts of steam. These volcanic earthquakes, 

 contrary to popular belief and to the ideas generally held for 

 many centuries by scientists, are in reality of but slight im- 

 portance and occur only in certain restricted districts. 



The recent eruption of Taal Volcano, January 27 to February 

 8, 1911, offered a very typical series of these earthquakes. On 

 the night of January 27, severe earthquakes occurred on Luzon 

 and in adjacent regions, while at the same time or very shortly 

 afterward it was noticed that the principal crater of the volcano 

 increased in volcanic activity. This activity as well as the 

 frequency and intensity of the shocks went on increasing during 

 the 28th and 29th, until at approximately 2.26 on the morning 

 of the 30th there took place the greatest and most destructive 

 eruption recorded in the history of the volcano. After this 

 paroxysm of eruptive activity the volcano returned to its normal 

 state in a short time, although the earthquakes continued to be 

 very frequent during the three following days, January 31 and 

 February 1 and 2, thus indicating that there was still an ac- 

 cumulation of energy in the interior of the volcano. During 



