230 STJtEAM TItf DEPOSITS OF TEEA.K. 



good deal of barren rock has been denuded since the rich beds at 

 the junction of the granite and clay have been washed away. 



To some extent, tin sand may have gravitated through the 

 loose watery sands even after they were deposited in beds. This 

 actually occurs in thin strata of washed sand which is thrown out 

 of the sluices. What little tin ore remains in this sand is found 

 to have settled down to the bottom. But, of course, this could not 

 happen through coarse gravel or compact clay. 



At the risk of being tedious, I must repeat the important les- 

 son to be learned from these facts. The way in which tin sand is 

 found in rich deposits in certain parts only of the drift, shows that 

 it has been the wearing away of some restricted portion of the 

 rocks. This is at the junction of a formation overlying the granite. 

 Wherever, therefore, either from the out-crop of the rocks or the 

 nature of the drift such a junction appears evident, deposits of 

 tin may be looked for. "Red claj^s are to be regarded as a special- 

 ly favourable indication, and so are out-crops of slate, schist or 

 limestone near granite. But an essential condition appears to be 

 that there should be high granite hills near, in order to secure the 

 requisite drainage for the formation of drift. 



I have mentioned how hollows in the ground affect the deposi- 

 tion of tin. There are a good many depressions of the kind about 

 these mines, though the surface is even. The ground, as the miners 

 say, rises up, and the ore is almost absent from the slopes, while it 

 is unusually rich in the hollows, those nearest the hills being the 

 richest. 



It may be asked whether tin sand might be looked for at any 

 great distance from the hills. To this a double answer may be 

 given. The first is that tin sand usually does not travel far, even 

 when it is very fine. A mile from its origin would be a long 

 distance. 



But, secondly, tin may be looked for far out in the plains, be- 

 cause it is certain that both paleozoic clays or granite in the form 

 of outlying hillocks have existed there, though now they are wash- 

 ed away. In this case, the nature of the soil would be the best 

 indication. 



The manner in which the paleozoic clays are stratified, and how 

 the strata are turned and twisted and crossed by white veins, has 

 suggested to the author of " Tin Mines in La.rut " that there were 

 fearful convulsions of nature going on when the stream tin was 

 deposited. But the cause of this dates much farther back. It 

 dates to the period when the paleozoic were affected by the gra- 

 nite, and crumpled or folded back by that rock. 



