100 ADAMS. 



of marine sediments similar to the marine conglomerates found on the 

 flanks of the Pico de Loro Mountains, but containing few bowlders and 

 some coralline reefs. On the strength of these observations, Santiago 

 Peninsula is mapped as occupied principally by younger coralline lime- 

 stones and marls. 



An area of coralline limestones also occurs southeast of the Loboo 

 Mountains and, as has been explained in another part of this paper, 

 these limestones are associated with some marls and probably grade into 

 tuffs. 



In making an excursion south of Lemery along the sea beach to 

 Point Ligpo, a limestone was found included by basalt and evidently 

 metamorphosed by contact with the igneous rocks into a very dense 

 stone. A peculiarity of this limestone not noted elsewhere in studying 

 the similar formations, was the presence of small cavities, or druses, 

 lined with clear quaftz crystals. The exposure occurs on the beach and 

 could not be followed very far inland. The peninsula was not thoroughly 

 explored. It may be that this limestone is a fragment of a tertiary 

 formation, but it seems more probable that it is a younger coral reef. 



Classes of igneous roclcs.- — Some petrographic work has been done upon 

 the rocks of this district, but it has been confined to a few specimens 

 collected during reconnaissance work. Oebbeke classed the rocks of Taal 

 as volcanic augite-andesite. Von Drasche recorded them as dolerite 

 with certain exceptions which . might be classed as augite-andesite. 

 Becker says that the active and exinct volcanoes of southern Luzon 

 appear to be mainly andesitic, but not devoid of basalts. 



A number of specimens were collected for petrographic study during 

 the field work which was carried on for this report. They have been 

 determined as andesites and basalts, but there is a gradation from one 

 class of rocks to the other. Apparently the basalts are the younger 

 rocks and they are found associated with the andesites in practically all 

 the eruptive centers of the district. 



Structure of the region. — There has been a tendency on the part of 

 former writers to consider the active and extinct volcanoes of this district 

 as forming a part of a chain extending into southeastern Luzon. This 

 is at variance with the structure of the island which, with the exception 

 of the Loboo Mountains, has a northwest trend. 



Banahao and its subordinate peaks lie. near the belt of folded tertiary 

 sediments which have a strike of 1ST. 30° W. and in so far as is 

 known there is no great fracture crossing the Tertiary formations. It 

 has been pointed out that Binangonan Peninsula, Talim Island, Mount 

 Maquiling, and Mount Malarayat form a chain of eruptives on an 

 approximately north and south line, or on two parallel lines having north- 

 west trends. Taal Volcano and the eruptive peaks which are grouped 

 around it are in strike with the axis of the central valley of Luzon. 



