STREAM TTK DEPOSITS OF PERAK, 229 



The red clays, and probably the yellow clays, are derived from 

 the paleozoic strata. The white clays may be decomposed fel- 

 spar from which the sand is washed out. All this careful sorting 

 and sifting has been effected by the force of gravity aided by the 

 never failing streams of water from the hills. 



Occasionally, vegetable soil is again repeated, showing that 

 there were different surfaces of dry land at different levels and at 

 various times in the geological history of these deposits. 



Then appear more or less worn fragments of quartz, felspar, 

 fluourspar, and granite. This may be called a gravel, but its 

 material is sometimes a stratum of mere pebbles, or sometimes 

 consisting of large boulders. These represent various vicissitudes 

 in the history of the stream. When such water- worn stones are 

 cemented together, the rock is called a conglomerate. 



Underneath all these deposits, at a depth of 20 or 30 feet, we 

 find the stream tin. It is usually in a gravel with much fine clay 

 and coarse sand, which gives the stratum a grey speckled appear- 

 ance. The deptli of the tin stratum is variable, but seldom more 

 than four feet, and often, in even rich mines, much less. The tin 

 rests upon white or blue clay either paleozoic or derived from the 

 granite. 



jNTow, when we find the tin sand all in one place and in the 

 lowest stratum, we must conclude that it came there by the force 

 of gravity, or that the upper part of the rocks from which the tin 

 was derived was richer in tin than that which subsequently sup- 

 plied the materials for the drift. Both these conclusions, I think, 

 are partly true. 



The drift overlying the tin may, in some case, have been re- 

 moved and replaced many times by the running waters as they 

 shifted their beds. Streams undermine their banks, they fall in, 

 and are thus turned over, washed and re-washed and the heavier 

 particles of tin soon become a stationary stratum in the lowest part. 

 This is the history of a good deal of the tin deposits, but not of all. 

 According to what has been already said, some portions of the ma- 

 terials for the drift were richer in tin than others, that is, the 

 junction of the paleozoic clays with the granite rock. When 

 these rocks were subjected to erosion, tin sand accumulated in 

 much larger quantities. 



If this explanation be correct, then we ought to find tin sand 

 at different levels in different mines, and, as a matter of fact, we do. 

 But in one group of mines there is generally a correspondence in 

 the level of the tin in all parts of the field. Thus in Thaipeng it 

 occupies nearly the lowest levels, from which we may infer that a 



