174 THE VOYAGE OF H.M.S. CHALLENGER. 



We ascribe the decomposition of this rock chiefly to the action of fumaroles. The 

 same explanation must also be given for the presence of gypsum associated with pyrites 

 at Zebu. Specimens of this mineral collected in that island show a compact and 

 whitish mass, sometimes laminated, and enclosed by a crystalline coating of pyrites ; 

 some of these crystals have the form of cubes, others of pentagonal dodecahedra. 

 Under the microscope the mass of gypsum appears as an aggregate of entangled 

 crystalline lamellae, which assume brilliant colours in polarised light. Some of the 

 sections show rectangular cleavages, and ought perhaps to be classed as anhydrite. 

 Colourless hexagonal sections with one optical axis, and presenting all the characters of 

 quartz, are to be seen in the microscopic preparations. These little crystals of quartz, 

 which are often associated with gypsum, are microscopic, perfectly colourless, and 

 contain liquid inclusions. 



We have attributed the alteration of these rocks and the formation of the secondary 

 products described above to the action of fumaroles. The effects of these emanations 

 are generally observed in volcanic regions, and in the Philippines they occur on a large 

 scale, for although, as stated above, there are no fumaroles of hydrochloric acid, those 

 charged with sulphuric acid are very numerous, and perfectly explain the products of 

 alteration we have described at Zebu. 



The action of sulphuric acid fumaroles on eruptive siliceous rocks should produce 

 gypsum, alum, hydrated aluminium, sulphate, and bianchetto, and according to the 

 intensity and duration of the action, the alumina is eliminated or converted into sul- 

 phate. The deposits of gypsum are here explained by the decomposition of minerals 

 of which lime is the base — hornblende, augite, and felspar, the presence of which in 

 the rocks of the island we have pointed out. The formation of pyrites is similarly 

 explained by the alteration of the iron-bearing minerals of the crystalline rocks. 

 Analogous phenomena are common in many other parts of the Philippine archipelago. 

 It suffices to recall that Mr. Semper has observed them at the sulphurous spring near 

 Maquilin, and Professor Roth cites a great number of localities where Dr. Jagor has 

 observed facts similar to those we have mentioned. 



The little island of Malanipa, where the few rocks about to be described were 

 collected by the naturalists of the Challenger, like Zebu, belongs to the Philippine 

 archipelago. It lies near Samboagan, bearing N. 66° W. from that island, and has an 

 altitude of 360 feet above sea-level. 1 The specimens examined are serpentinous rocks 

 derived from the decomposition of peridotites. 



One fragment of serpentine is traversed by veins of chrysolite ; the rock itself is 

 black and shining, spotted with green. Dark particles 3 to 4 millimetres in diameter, 



1 Narr. Chall. Exp., vol. i. p. 605. 



