STREAM TIN DEPOSITS OF PERAK. 235 



tory of the granites. First of all they are generally very old rocks. 

 I say generally, because though most granites are paleozoic, there 

 are mesozoic or secondary and cainozoie or tertiary granites as 

 well. Bat the rock ve have to deal with here is paleozoic, and I 

 will consider that as affording the simplest case for consideration. 



Now, we have evidence iu this country that the granite here 

 has been covered by two more formations at least. These were of 

 considerable thickness. Fifteen hundred feet of limestone is ex- 

 posed in places, and even then it has been greatly denuded or worn 

 away. The paleozoic clays belong to a formation which is known 

 everywhere on the earth's surface to be very thick. The history 

 of the geological changes in the earth's crust justifies the inference 

 that between these two formations and the comparatively recent 

 date of their uncovering and denudation, many other formations 

 must have succeeded and disappeared. So that, without any stretch 

 of imagination, you can perceive that our granite was at one time 

 covered by an enormous weight of overlying rocks. The pressure 

 thus effected I do not attempt to estimate. It defies calculation. 

 Millions of tons weight would result from a hundred feet or so of 

 rock, so what of thousands of feet ! 



Now, pressure engenders heat. If we cannot estimate the 

 weight, we may say, that at the most moderate computation, the heat 

 engendered by pressure would have been sufficient to liquify the 

 rocks. But the pressure would prevent liquif action. The nature 

 of the overlying rocks would also prevent much of the heat being 

 lost by radiation. 



Let us turn for a moment, before we consider the effects of 

 this heat, to take into account the material with which it has to 

 deal. Before these granites were covered over, they were stratified. 

 We can see this in many places where the marks of stratification 

 have not been obliterated. There was a time, "then, when these 

 strata were laid down line by line horizontally by the river or the 

 sea or the aerial current from which they were deposited. They 

 then consisted of sand, which means silica, of mud or clay, which 

 means alumina, magnesia, lime, soda, potash and a little oxide of 

 iron. Fluorine and carbons, tin, gold or silver were also present 

 infinitesimally. How they came to be present, I shall explain 

 hereafter. 



But there was one very important ingredient which we must 

 not leave out, and that was water. All rocks contain this in a cer- 

 tain proportion. I do not mean those stores which come out as 

 springs upon the surf ace, but water mingled with the ingredients of 

 the rocks, that is, chemically combined. Grases of course there were, 



