506 E. S. Willboum — The Pahang Volcanic Series — 



Under the microscope the lava fragments are seen to be of two 

 varieties, one made up entirely of felspar laths with trachytic 

 structure, the other being very fine-grained and containing spherulites. 

 The felspar crystals also are of two varieties — oligoclase-andesine 

 and orthoclase. There is a good deal of dark-green chloritic material 

 spread throughout the rock. 



A similar rock at Sibah near Kuala Lipis contains a good deal of 

 secondary epidote and mica, some of which has been formed by the 

 alteration of augite crystals. No augite is left unaltered, but the 

 sharply defined octagonal outline of the pseudomorphs suggests that 

 augite was the original mineral. 



Andesite tuffs without considerable admixture of more acid 

 material are rare, but a specimen of greenstone with abundant 

 epidote collected on the Benta-Kuantan road near the main Gondwana 

 outcrop east of the Pahang River is more basic than those hitherto 

 described, and contains little, if any, rhyolite material. The fragments 

 of andesite-lava contain small augites as granules interstitial to the 

 felspar laths. Crystals of oligoclase-andesine and augite are common, 

 and there are a few fragments of quartz which are penetrated by 

 epidote, and which when viewed between crossed nicols look like 

 fragments of granophyre in which the felspar has been replaced by 

 epidote. Fragments of an altered shaly sedimentary rock occur, 

 and so do occasional rounded grains of corundum. Magnetite is very 

 abundant in this rock, most of it being secondary. 



Besides the above types, consisting in the main of fragments with 

 very little fine-grained matrix, there is the compact type of tuff, in 

 which the bulk of the rock is made up of a very fine-grained material, 

 now of a siliceous nature. 



Such deposits can only be determined as volcanic when a con- 

 siderable number of the larger fragments are of felspar or, as only 

 rarely happens, when fragments of lava can be recognized; in fact, 

 there is an insensible gradation into a sedimentary grit. Probably 

 many rhyolite tuffs are of this nature, and cannot be distinguished 

 from felspathic grits. Then, again, other rocks very much resemble 

 rhyolite tuffs, but they cannot be distinguished from partly altered 

 rhyolite-lava flows unless some field evidence is forthcoming. 



Deposits of Boulders in Tuff. 



A most remarkable deposit is found in Pahang associated with 

 tuffs and breccias similar to those already described. It consists of 

 rounded masses of lava, tuff, acid intrusive rocks, or sedimentary 

 rocks embedded in a fragmental matrix, the masses being very 

 different from bombs and lapilli which are usually associated with 

 tuffs. In several of the known exposures the boulder-in-tuff deposit 

 passes into the usual tuff without boulders or pebbles. 



Their characteristics are summed up as follows: — 



1. Thev vary in size from a pea to over a yard across (at Kuala 

 Tekal). 



2. They are water-worn, show no sign of vesicular structure, and 

 have not a glassv margin. 



