164 IDDINGS. 



intersertal matrix of globulitic glass. The glass and crystals are in nearly equal 

 proportions, and the average diameter of the crystals is about 1 millimeter. 



Somewhat similar basalt with much globulitic glass in the groundmass occurs 

 on Palawan Island. 



( )n Masbate some of the volcanic rocks differ notably from most of 

 those found elsewhere in the islands, in that they appear to contain higher 

 amounts of potash, which shows itself in orthoclase, mica, and what was 

 probably originally leucite. It is possible that related rocks will be found 

 in other parts of the Archipelago when it is more thoroughly explored. 

 These rocks have undergone considerable alteration and their exact com- 

 position is not readily determinable from the sections studied. 



The rock from about 2 kilometers southwest of Aroroy is dark-gray, sempatic, 

 mediophyric, and seriate; the diameters of the phenocrysts varying from 5 milli- 

 meters to less than 1 millimeter. They are chiefly euhedral twinned augite, 

 greenish with faint pleochroism, from green to yellow; clusters of equant colorless 

 crystals, and some isolated euhedrons of a mineral which appears to be leucite, 

 possibly altered to analcite, as it does not exhibit birefrigence. There are some 

 small phenocrysts of plagioclase, partly altered and probably alkalic, or ealci- 

 alkalic. The augite phenocrysts are zonally developed, with "hour-glass" structure 

 in some cases. They contain glass inclusions and small crystals of magnetite. 

 Surrounding the phenocrysts, especially the augite, there are dark clusters of 

 minute crystals that extend to various distances into the groundmass. They are 

 needles and minute prismoids of augite in subparallel clusters. In places they 

 seem to be granulated, or coarsely globulitic. With these prismoids are mingled 

 opaque needles, or blades of what appears to be limonite; but from their resem- 

 blance to similarly shaped and arranged blades of brown mica, present in closely 

 related rocks of this locality, they may be paramorphic mica. They are in sets, 

 or groups, of parallel prismoids, often at different angles in the matrix, or in 

 fern-like arrangement; a set of minute, parallel blades being crossed by a 

 larger one like a stem. 



The cluster of colorless minerals have the form of leucite crystals, are euhedral 

 to subhedral ; and in some cases rounded. Some carry minute inclusions, centrally ■ 

 located, less often zonally. In several finer-grained varieties of the rock there 

 are minute, imperfect, skeleton forms, characteristics of leucite. The groundmass 

 consists of indistinctly outlined, clouded feldspar, possibly orthoclase in parf, 

 besides products of alteration. In the finer-grained variety the prismoid and 

 needle-like alkalic plagioclase is more distinct. There are prismoids of augite, 

 and groups of parallel needles or blades of pale brownish, pleochroic mica. 



Another rock from about 1 kilometer southwest of Mount Bagadilla is like the 

 one just described in the habit of the groundmass, is but slightly porphyritic, and 

 contains less ferromagnesian. minerals. There are groups of minute blades of 

 mica in parallel arrangement, and a few imperfect skeleton forms of leucite. The 

 rock is partly altered. 



The rock from the south side of the Guinobatan River, south of Mount Baga- 

 dilla, is an altered rock like the one just described. A coarser-grained variety 

 from the locality last mentioned is slightly porphyritic, with shreds, or blades, 

 of light brown mica in diverse arrangement, and with a matrix of clouded 

 orthoclase. It forms a transition between the leucitie rocks just described and 

 the syenite. It is also somewhat decomposed. 



