GEOLOGY OF THE PHILIPPINE ISLANDS. 327 



is soft, but it gradually hardens on exposure. It is buff colored to gray. 

 It was extensively used in the Spanish regime for building purposes. 

 Fragments of pumice, black hornblendes, fragments of feldspar, etc., 

 give the rock a very heterogeneous composition. The maximum thick- 

 ness of the deposit is probably more than 100 meters. The best exposures 

 are to be seen in several quarries along the banks of the Pasig Eiver. 



5. Cherts. — In Ilocos Norte I found a few outcrops of a hard, red, 

 fissile chert, which on examination proved to have fragments of the tests 

 of Radiolaria. These are very similar to some cherts of the Moluccas 

 described by K. Martin and assigned to the Jurassic. 19 



As regards distribution, the general statement may be made that the 

 oldest rocks probably are those found in the deep canons of the Cordil- 

 leras of Luzon. In many other parts of the Islands where we might 

 hope to find them we encounter everything covered by a sheet of volcanic 

 rocks, as is the case with much of the western part of Mindanao, or else 

 by a mantle of coral limestone, as in Cebu. Flanking these older rocks, 

 and dipping away from them both to the east and west, are the Tertiary 

 sediments, limestone, sandstone, shale, and the intercalated coal seams; 

 above these are andesitic and basaltic flows, while the youngest consol- 

 idated formation of all is the tuff of the vicinity of Manila. It is not 

 easy, in our present state of knowledge, to delimit all of these formations ; 

 indeed, manjf which appear to be of different age are in reality contem- 

 poraneous. Another noteworthy fact is particularly well exemplified in 

 Cebu, namely, that there is no apparent break in the limestone from 

 the coral reef on the shore to the capping of the cordillera in the center 

 of the island, at a height of 1,000 meters. I have walked over a limestone 

 formation on that island, which is continuous from the living coral reefs 

 to the Pliocene, and probably the Miocene, with apparently no un- 

 conformity. This island must have suffered a long period of erosion 

 after the Miocene. It probably sank below sea level and subsequently 

 rose so gradually that the whole island was covered with a mantle of 

 coral limestone. This mantle has since largely been removed by erosion. 



GEOLOGIC HISTORY. 



In taking up this part of my discussion, I feel that the dominant 

 episodes in the geologic history are best given in Becker's admirable 

 summary and by adding such comments as later and more intimate 

 acquaintance with the field shows to be necessary. Mr. Becker says : 



It would seem that the geological history of the Philippines is something as 

 follows: From early Paleozoic times onward an archipelago has usually marked 

 the position of these islands. Prior to the Eocene nothing definite is known of 

 them, but further investigation will very likely disclose Paleozoic and Mesozoic 

 strata there, as in the Sunda and the Banda islands. During the Eocene it is 



"Reisen in den Molukken, etc. Geol. Theil. Brill, Leiden (1902), 170. 



