fgifi.l ScRIVENOR on Pulau Pisang Rocks. 33 



There can be no doubt that these rocks are from a junc- 

 tion of Chert Series rocks and granite and there are two points 

 concerning them that are worth attention. The first of these 

 is the association of volcanic ash. 



The coarser specimens cannot be distinguished from some 

 of the ashes of the Pahang' Volcanic Series, and this is the 

 third instance in which these volcanic rocks have been found 

 associated with chert. Another instance is at Lubok Plang, on 

 the Pahang River, where a bed of chert was found between a 

 flow of lava and a layer of ash. Epidote occurs in the chert 

 and in the volcanic rocks but in the Pulau Pisang chert it is 

 more abundant. Radiolaria are more abundant in the Lubok 

 Plang chert. 



The second known instance of the association of volcanic 

 ash and chert was afforded by specimens taken from a stone- 

 heap in Singapore. The radiolaria in the chert are in some 

 cases at any rate preserved as casts of chlorite and the same 

 mineral occurs in the ash. Nothing definite could be learned 

 about the locality whence these rocks came. One statement 

 was to the effect that they might have come as ballast from 

 Mauritius, but that is very unlikely, and in view of the nature 

 of the Pulau Pisang rocks, they may have come from a neigh- 

 bouring island.* 



Generally the radiolarian cherts are found close to thick 

 beds of quartzite and shale, and in the coarser quartzites 

 pebbles of chert are abundant. Lately Mr. E. S. Willbourn 

 has reported chert and quartzite to be interbedded in certain 

 sections in Negri Sembilan. These three cases of ns^ociation 

 with igneous rocks suggest that in some cases their origin may 

 be the same as that put forward in the Geological Magazine 

 for 191 1 (British Pillow Lavas and the rocks associated with 

 them — loc. cit. pp. 202-209 and 241-248) by Messrs. Dewey 

 and Flett, who think that silicate of soda from volcanic 

 eruptions was dissolved in sea-water and created conditions 

 favourable for siliceous protozoa such as radiolaria. A^ the 

 eruptions that formed the Pahang Volcanic series were in part 

 submarine, this may be a case of similar conditions ami similar 

 results. 



The other point of interest is the resemblance of the pale 

 grey rock (Nos. 7 & 10) to some of the boulders and pebbles 

 found in Kinta with the boulders of tourmaline-corundum 

 rock. These are light colored, sometimes oolitic, and some- 

 times contain a little corundum and tourmaline. In a descrip- 

 tion - !" of the tourmaline-corundum rocks it was suggested that 

 certain bodies in them may be replacements of casts of 

 radiolaria, and a rock was found in Kinta actually showing 



• A fourth occurrence of chert associated with an igneus rock fs known 

 on the Ginteng Sempah Road, Selangor. 



t Quart Journ Geol Soc. Ixvi 1910, pp. 435-449. 



