52 WARREN D. SMITH 
There is complete gradation from this rock into andesite, the differ- 
ence between the two being chiefly one of depth and hence rate of 
cooling of the original magna. 
As Professor Iddings has worked over most of the rocks in our 
Philippine collection, I shall refer the reader to his descriptions of 
these rocks. 
©s 1SIDRO 
~~ 
a 
O85 ¢ 5. Miguel 
Lege 
Silidefonso 
2 S=\Maria 
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Sk 
OLE] Qi apne 
Manila Bay [ pay Ny 
CAVITE Qf" 8 ‘ 
2 Alyeanrst 
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Fic. 7.—Map of Southwestern Luzon. (After Centeno.) 
The extrusives.—As far as we now know, the andesites form the 
more or less worn-down volcanic stocks, and the basalts represent 
distinctly later flows. The andesites and basalts are generally 
fairly uniform, but we have besides, vast areas of volcanic agglomer- 
ates grading from a formation made up of large blocks more or less 
angular and somewhat scoriaceous, to a rather fine-grained tuff. 
tJ. P. Iddings, “ Petrography of Some Igneous Rocks of the Philippines,” Phil. 
Jour. Sci., V, No. 2, 155. 
