Vol. 68.] THE GOPENG BEDS OF KINTA. 147 



This also might render small pre-existing quartz-veins unrecogniz- 

 able. 



Many geologists have doubtless noticed cases of selective meta- 

 morphism by igneous rocks : the limestone, the Gopeng Beds, and 

 the phyllites and quartzites here show an interesting difference in 

 the effects of regional metamorphism. 



The Petrology of the Gopeng Beds. 



Leaving one item, namely the presence of cassiterite, for con- 

 sideration later, the petrology of the clays and boulder-clays may 

 here be briefly described. 



A detailed knowledge of the mineral constitution of the clays was 

 obtained by washing, and examining under the microscope and by 

 other methods, numerous samples from the bores. The clays 

 always contain some sand, which may vary from a quantity so 

 small as to be difficult to estimate to 30 per cent, or more. The 

 clayey material, as already indicated, is largely composed of hy- 

 drated silicate of alumina, and free aluminium hydroxide. The 

 sand is mostly composed of quartz, but tourmaline, topaz, ilmenite, 

 and magnetite frequently occur, the first two constantly, in my 

 experience, wherever tin-ore is abundant. In addition to the 

 minerals just enumerated, zircon and manganese dioxide are found : 

 also corundum and monazite as occasional grains (in the neighbour- 

 hood of Gopeng); at Pulai, corundum in small grains is very 

 abundant in association with the roughly washed tin-ore. 



The matrix of the boulder-clays has the same mineral con- 

 stitution as the clays. The following is, T believe, a complete list 

 of the boulders found so far : — 



Quartz : abundant. 



Sandstone. 



Sandy schists with white 



abundant near Gopeng. 

 Phyllite: rare. 



Tourmaline-schist : abundant, and 

 often veined with quartz and white 

 mica. The tourmaline is brown. 



Granite : not common at Gopeng, 

 but common at Pulai. 



Hornstone : rare. j TourmaHne-granite : .Do. do. 



Hornblende-schist: rare. ! Corundum : rare at Gopeng, abundant 



Tourmaline-rock: abundant. : in one part of the Tekka Ltd. pro- 



Quartz-muscovite rock : abundant. I perty, and abundant at Pulai. 



Quartz-tourmaline-muscovite rock : Cassiterite : rare as boulders. 



abundant. 

 Quartz-kaoHn rock : common. 

 Small masses of kaolin : not veiy 



common. 



White mica. Probably formed a vein- 

 stone originally. 



The boulders differ considerably in size and abundance in the 

 clay. They vary from small pebbles to masses weighing over 50 lbs. 

 Exceptionally large boulders are occasionally found, the biggest 

 that I have seen being exposed at the New Gopeng Mine. It is 

 composed of quartz and tourmaline, and, judging by the measure- 

 ments, should weigh about 3-6 tons. 



Some of the boulder-beds are crowded with boulders to such an 

 extent. that the matrix of clay only occupies interstices between 



