GEOLOGY OF THE PHILIPPINE ISLANDS. 325 



the norms of a basalt and a gabbro are not essentially different and the 

 same is true of andesites and diorites. The older classifications did not 

 show this. 



As would be expected from the prevalence of basalts in the Archipelago, 

 gabbros are quite common. An olivine-gabbro from river gravel at Mon- 

 talban, Luzon, contains labradorite, Ab 2 An 3 , pale green augite, colorless 

 olivine and very small amounts of secondary hornblende, "uotite and 

 magnetite. There also are some secondary minerals locally developed 

 such as chlorite, serpentine, etc. 



A very fine-grained norite is found on Palawan. 



Gabbros with an ophitie texture, that is, with the component minerals 

 forming a lattice-like structure, often called diabase or dolerite, are 

 numerous. To avoid a multiplicity of names which only confuse and 

 lead us away from the true conception of the relationships of these rocks, 

 I shall henceforth drop these two terms and keep the name gabbro. 



6. Peridotite. — When a gabbro contains much olivine it is customary 

 to call it a peridotite. 18 While this rock is not common in the Islands 

 there are several occurrences of it. Ambos Camarines is one of the 

 localities where it is found. 



7. Granite is a rather rare rock in the Philippines. So far as I know, 

 the best development of granite in the Islands is in the Paracale-Mam- 

 bulao mining district, Ambos Camarines. It has been so squeezed 

 during the regional metamorphism of the district that it now has a 

 gneissoid texture. 



Iddings, who visited the district with me, says the granite of Mam- 

 bulao has been sheared to a thinly laminated gneiss with "Augen" 

 structure on a small scale. The orthoclase and albite lie as anhedral 

 blocks in a matrix of smaller equant anhedrous (shapeless) quartz and 

 orthoclase with shreds of muscovite (and chlorite, it should be added) 

 having pronounced fluxion structure. ' 



8. Syenite.- — A sodic syenite has been found in Masbate. I have 

 seen nothing else which in character even approaches this rock. 



In concluding bis paper Iddings says : 



The extremes of this mineral variation, or differentiation in this region, appear 

 to be albitic granite and albitic syenite on the one hand, and peridotite and 

 possibly pyroxenite on the other. 



There are not sufficient data at hand to determine the relative abundance of 

 the different magmas and so indicate the composition of the average or "parent" 

 magma. It appears that the coarse-grained, intrusive rocks have attained a 

 higher degree of differentiation than the extrusive lavas, but this may not be 

 actual fact, and further study of the region may modify this conclusion. 



Sedimentary rocks. — This group includes the usual general classes to 

 be found in any part of the world. The following list shows the order of 



18 From the French world peridot for olivine. 



