TIN DEPOSITS OF THE MALA Y PENINSULA 1 43 



At the bottom of the alluvium is generally either granite or 

 limestone, though frequently where the tin stops, barren alluvium 

 or rock decayed in situ separates it from the bed-rock, so that the 

 solid granite or limestone is not always seen. This is especially 

 true in granitic areas where the surface of the rock below the 

 alluvium is often altered to a soft, partly kaolinized mass. 

 Sometimes beds of coarse granite pebbles and bowlders, forming 

 the substratum of the tin alluvium, have decayed in situ in the 

 same manner as the surface of the original rock ; and it is not 

 uncommon to see rounded granitic fragments converted into a 

 soft putty-like mass, which when broken up gives rise to angular 

 particles of the original quartz of the rock and a soft clay 

 resulting from the decay of the feldspar. Hence angular quartz 

 may often be found in deposits that have been transported long 

 distances. Such an occurrence is seen on the property of the 

 GopengTin Mining Co., where the tin-bearing stratum consists of 

 a more or less ferruginous deposit of sandy and gravelly material 

 occupying a ridge on the side of a small stream and underlaid by 

 a pebbly stratum like that just described. In the creek bed 

 below, near the native town of Gopeng, tin alluvium washed 

 down from the ridges is extensively worked by the Chinese. 



The limestone bed-rock is often leached in deep hollows and 

 caves, as seen at Chongkat Pari and near Tronoh (see Figs. 4 

 and 6), while elsewhere, as seen between Ipoh and Lahat, it 

 forms an undulating surface with alternating protrusions and 

 recessions, following regular lines, probably influenced by lines of 

 bedding, and resulting in a series of natural riffles behind which 

 cassiterite has concentrated. (See Plate IV.) This occurrence 

 is similar to the way gold has collected behind limestone riffles 

 near Columbia, in California, and the country has been stripped 

 in search of ore in much the same way as in the California 

 region. The road from Ipoh to Lahat runs through a broad val- 

 ley, and the rough surface of the bare limestone bed-rock is 

 seen in many of the old workings. 



Tin alluvium frequently occurs on ridges and hills as well as 

 at lower levels in the valleys and creek beds. This sometimes 

 suggests strongly that, since the formation of tin alluvium began. 



