10 A SKETCH OF THE GEOLOGICAL STRUCTURE 



altered and hardened by the adjacent granite of Pulau Ubin, 

 which we will refer to as the Mesozoic granite, as being part 

 of the granite whose history we have just traced. The lavas 

 and ashes of Pulau Nanas then are certainly older than the 

 Mesozoic granite, but if we examine the ashes on the north 

 coast of the island closely we shall find that it contains 

 angular fragments of granite that have been altered in the 

 same manner as the ashes and lavas. Now we know that 

 granite is not a volcanic rock : no volcano has ever been 

 known to pour out granite as lava, and no volcanic ash 

 resulting from the explosive force of steam on viscous . granite 

 has ever been found. But it is not uncommon to find in 

 volcanic ashes portions of the wall of the vent by which the 

 lava forced its way to the surface or the sea-bottom, and the 

 simplest explanation, if not the only one. of the presence of 

 those granite fragments in the Pulau Nanas ash, is that the 

 vent passed through granite forming part of the sea-bottom, 

 or perhaps in this case, of the land surface. Therefore before 

 the Carboniferous volcanic rocks were poured out, an older 

 granite mass had solidified. We will refer to this as the 

 Palaeozoic granite. 



Before leaving the neighbourhood of Singapore there is 

 some more information to be noted in connection with this 

 Palaeozoic granite. Gondwana rocks are largely exposed 

 here and in a conglomerate bed I found two small pebbles of 

 a rock composed of quartz and tourmaline. There are no 

 rocks known in the older calcareous series from which such 

 pebbles could have been derived, unless they be ashes such as 

 those at Pulau Nanas containing fragments torn from a 

 volcanic vent, but even in that case we should be obliged to 

 regard those pebbles as derived ultimately from a mass that 

 existed before the calcareous rocks were formed. As our 

 knowledge of the Mesozoic granite leads us to expect quartz- 

 tourmaline rocks to be connected with granitic rocks, we 

 therefore suspect a possible connection between those pebbles 

 and the fragments of Palaeozoic granite in the Pulau Nanas 

 ash. 



Jour. Straits Branch 



