REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 171 



fusion marks the place where they are united to the eruptive rock ; the quartzite, 

 assuming a darker colour, passes insensibly into andesite. The alkalies present in the 

 andesite doubtless acted upon the silica of the quartzite to produce this zone of fusion. 

 The embedded fragments measure 4 or 5 centimetres ; some smaller specimens were seen, 

 but they have almost entirely fused, assuming an opaline appearance. Microscopic 

 examination shows that, except in the zone of fusion, these quartzites are made up of 

 irregular grains of quartz, without any amorphous matter. Very small greenish 

 crystallites grouped in gerbs or fans, and imbricated scales of tridymite, are observed 

 in the quartzites. 



B. Racks of Zebu and Malanipa Islands. 



The few specimens from these two islands of the Philippine group which we 

 will describe were collected by Mr. Buchanan in the course of a hurried exploration, 

 and they represent some only of the lithological types which are characteristic of these 

 islands. The specimens deserve attention, because these localities are rarely visited 

 by geologists, and because the rocks allow us to extend to these islands, with great 

 probability, the interpretation admitted for the larger islands of the group, regarding 

 the schisto-crystalline nature of the archipelago, and the presence of ancient eruptive 

 rocks. 1 These researches also allow us to generalise another order of phenomena, which 

 has been observed in other islands of the group, viz. the alteration of volcanic rocks 

 by the action of sulphurous emanations. It is well known that no fumaroles containing 

 hydrochloric acid have been observed in the larger of the Philippine Islands, while 

 sulphurous fumaroles play a considerable part in the decomposition of rocks in that 

 locality. We shall see that the massive eruptive rocks of Zebu have undergone the 

 action of sulphurous vapours like those of all other parts of the archipelago. 



The island of Zebu, famous for the death of Magellan, has been long known to 

 naturalists, since it is almost the only locality where the beautiful siliceous sponge 

 Euplectella aspergillum was formerly dredged. Zebu is 120 miles long, from 10 to 17 

 miles in breadth, and has an area of about 1200 square miles. It is traversed from north 

 to south by a chain of mountains, and contains deposits of lignite which are being worked. 2 



The rocks to be described were collected in the neighbourhood of the town of Zebu, 

 where they are exposed in the bed of a river. One of them is a greenish black 

 fine-grained specimen ; little lamellae of plagioclase are seen sparkling, with the naked 



1 Mr. T. E. Tenison-Woods has published a resume of his researches on the geology of Malaysia, the south of China, 

 &c. (see Nature, vol. xxxiii. p. 231, 1886). His conclusions with regard to the nature of the geology of Malaysia and 

 the Philippines agree closely with those put forward by Professor Roth in the appendix to Jagor's work, and with those 

 derived from researches on some rocks from the island of Camiguin. The vast region examined by Mr. Tenison-Woods 

 presents a remarkable uniformity in geological structure. Granites and intrusive rocks form the lower masses, and are 

 covered by palieozoic schists and slates. In some places beds of limestone, probably carboniferous, appear, and finally 

 deposits of coal belonging to different formations. Marine deposits of miocene and pliocene age were also observed. 



2 For the age of the coal and lignite beds of the Philippine Islands, see Tenison-Woods, loc. cit. 



