REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 173 



Another rock from the same locality showed on examination a composition and 

 structure identical with that just described. The one detad to note is that epidote was 

 found in yellowish grains included in the felspar. Although this mineral plays a 

 purely accessory part, its presence has a certain significance, in relation to the 

 determination of the age of the rock in question. 



At first sight one is tempted to refer these rocks to basalt, for they have the same 

 composition and structure, but on taking their mode of decomposition and the 

 presence of epidote into account, it seems more natural to class them with the rnela- 

 phyres and peridotic diabases. It is known besides, as pointed out in speaking of 

 the rocks of Camiguin, that palseo-volcanic masses are represented in the Philippines. 

 There is nothing surprising, therefore, in finding rocks of the diabase family on this 

 island. We must, however, add that this determination as palseo-volcanic rock cannot 

 be established with certainty in the case under consideration so long as there are no 

 stratigraphical data to found upon. 



We shall now describe the altered specimens and the secondary products formed by 

 the action of fumaroles. One of these decomposed rocks is formed of a mass of 

 whitish grey clay with a greenish tinge ; it is friable, and may be scratched by the 

 nad. The naked eye distinguishes small bright crystals of pyrites, and sometimes 

 milky grains of felspar. The specimen is covered in some places with a coating of 

 limonite, and gives out a strong argillaceous smell. Microscopic examination shows 

 that the alteration has principally affected the ground-mass and the bisilicate, which 

 must formerly have been a constituent, and has now entirely disappeared, giving rise 

 to chlorite surrounding all the elements. The felspar is sometimes transformed into 

 saussurite, granules and characteristic needles of which are found in the plagioclastic 

 sections. The plagioclase is still fresh enough in some cases to show hemitropic 

 lamellae according to the albite law, and the primitive outline of this mineral may 

 sometimes be traced out. In a section parallel to M traces of the faces PyT are 

 seen, and the cleavage parallel to P, and also those of the prisms less marked. It is 

 thus possible to estimate the angle of extinction accurately enough, and the mean of 

 observations gave + 20° for the plagioclase. This felspar thus approximates a mixture 

 of oligoclase and albite. The rock may be classed with diorites rich in felspar, if we 

 admit, as is probable, that the bisilicate was formerly represented by hornblende. It 

 is well known that the presence of oligoclase has often been proved in rocks of this 

 type, and even albite has been observed in diorites. Epidote, of which some grains are 

 occasionally found, also leads to this determination. 1 Numerous sections of pyrites, 

 also a secondary mineral, are frequently observed. 



1 We must note that epidote is found in recent eruptive rocks, for example, in amphibolic andesite (compare 

 J. Roth, Chem. Geol., p. 351), but it is no less true that this mineral is comparatively rare in the crystalline masses 

 of that age, whilst it abounds in the older amphibolic plagioclastic rocks. 



