314 SMITH. 



ORIGIN AND AGE OF THE SEVERAL TYPES. 



Mount Mayon is an ash cone. Any violent eruption now would 

 probably destroy its perfect outline. Its present slopes bear a definite 

 relation to the angle of repose of the materials which compose it. 

 Volcanoes of this type are considered to be of very recent age, erosion as 

 yet having had but little effect on them. Mayon may be just in its 

 prime. 



Mount Mariveles is a type of a volcano which has passed into old age. 

 Activity has long since ceased and erosion has made great inroads on it. 

 Its great crater lias been broken through entirely on one side and a 

 great canon leads out from it toward the sea. This mountain probably 

 is a Pleistocene volcano. Both andesite and basalt, chiefly the former, 

 and but little ash are found on it. 



Taal Volcano consists of one moderate sized crater, 2,000 meters 

 across and 270 meters deep, on a low island in the center of Lake Bombon, 

 28 kilometers wide, situated 62 kilometers from Manila. Several smaller 

 craters and pools of hot water occur within this large crater. There is 

 only one active vent near the center; where a small column of steam and 

 gases issues. 



Becker's idea of the origin of Taal was that the mountain slopes once 

 continued upward along a hyperbolic curve, as in Mayon, to an elevation 

 perhaps of over 4,000 meters and that a great explosion and subsequent 

 sinking of what was left caused the present condition. 



However, Adams' 8 theory does not consider an explosion at all, nor 

 does it presuppose as high a cone as must have existed if we grant 

 Becker's view to be correct. He accounts for the formation of Lake 

 Bombon by peripheral faulting with subsidence of the central area. 



There are several minor crater lakes in the Philippines, but that of 

 Taal is the largest and best known. Taal was last in extended eruption 

 in 1904. 



STRATIGRAPHY. 



Table 1 gives a tentative scheme of the stratigraphy of the Islands and 

 shows that we have no sedimentary formations known with certainty to 

 be older than the 'Eocene; indeed, it is not positively established that 

 there are any sediments older than the Oligocene. Nummulites were 

 reported by Abella ° and Eichthofen. 10 In their type localities abundant 

 orbitoidal forms have been collected, but no nummulites. • In the 'lower 

 "limestone of Batan Island, Douville found one form which he determined 



8 Geological Reconnaissance of Southwestern Luzon. This Journal, Sec. A 

 (1910), 5, 57. 



,J Isla de Cebu (1886), 109. 



10 Ztschr. d. geol. Gesell. (1862), 14, 357-360. 



