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fgifi.l ScRIVENOR on Pulau Pisang Rocks. 33
There can be no doubt that these rocks are from a junction
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 stoneheap
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 neighbouring
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 description
-
!" 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.