﻿"Vol. 66.^ PULAU UBIN ANB PULATT NANAS. 433 



these gabbroid rocks in the geology of the East Indies. Dr. Yerbeek 

 has described a series of lavas in Amboyna which he terms 

 ambonites : they vary from acid to basic, and are characterized by 

 the presence of bronzite, cordierite, and garnet. Tn his publication 

 of 1905, 1 Dr. Yerbeek argued at length in favour of the basic rocks 

 being Cretaceous, but in this opinion he differed from Prof. Martin. 

 In a later publication, however, Dr. Yerbeek admits that the 

 ambonites may belong in part to the older Tertiary. 2 Nevertheless, 

 his opinion would appear to be that, about Cretaceous times, certain 

 lavas bearing rhombic pyroxene were erupted on what is now 

 Amboyna. 



Prof. Molengraaff, again, 3 describes gabbro and norite from 

 Borneo, and says that the gabbro is almost certainly younger than 

 the ' Danau ' formation, a series of rocks probably Jurassic in age. 4 

 In Sarawak I have found norites and hypersthene-andesites whose 

 age I was unable to determine with certainty, but which are 

 younger than the Middle Jurassic. 



In themselves the rocks of Pulau Ubin afford a very interesting 

 problem in petrography. To one whose aim in scientific work is to 

 effect a junction with the Dutch geologists on the one hand, and 

 the Indian geologists on the other, it is a great advance to find 

 such evidence as the foregoing pointing to a correlation with 

 rocks bearing rhombic pyroxene in Borneo and Amboyna ; but it 

 is most interesting to note that in Cretaceous times, and perhaps 

 earlier, before the great tin-bearing granite masses of the Peninsula 

 had consolidated, a basic magma had already been differentiated, 

 capable of providing the vast outpourings of volcanic rocks that 

 have continued so late as the terrific eruption of Krakatoa in 

 1883. 



YI. Conclusions. 



The foregoing remarks may be summarized as follows : — 



(1) The normal granite of Pulau Ubin is hornblende-granite, 

 the age of which is certainly post-Triassic and pre-Eocene, perhaps 

 post-Inferior- Oolite and pre-Cretaceous. 



(2) Veins of quartz-norite and masses of quartz-biotite-gabbro, 

 and veins and masses of a fine-grained rock which maybe described 

 as enstatite-spessartite, are found in the normal granite of Pulau 

 Ubin. These point to an early differentiation of a granitic and a 

 gabbroid magma, perhaps in pre-Cretaceous times, and they are 

 referable to rocks in Borneo and Amboyna. 



1 ' Description Geologique de l'lle d'Ambon ' pp. 101-105. 



2 ' Rapport sur les Moluques — Reconnaissances Geologiqnes clans la Partie 

 Orientale de FArchipel des Indes Orientales Neerlandaises ' Edition franchise 

 du Jaarb. v. h. Mijnwezen in Nederl. Oost-Indie, vol. xxxvii (1908) partie 

 scientifique, p. 769. 



3 ' Geological Explorations in Central Borneo (1893-94)' 1902, pp. 433-34. 

 ^ Id. ibid. p. 414. 



