STREAM TIN DEPOSITS OF PEEAK. 237 



cal features. This fact must also explain the presence of metals 

 in one portion and not in another. 



Some persons might find it difficult to understand how any in- 

 terchange would go on under such pressure, but it will solve the 

 difficulty to some extent when they are reminded that interchange 

 and chemical action goes on in the hardest and most solid rocks. 

 Solidity is a relative term. There is nothing on the earth so hard 

 that a movement is not going on in its particles. The moving 

 crystals in the cavities of granite prove this. Some think that light 

 is the stimulus in this case. It may be so. That shows how even on 

 the hard transparent diamond movement is continually going on, 

 movement that is not more appreciable than the waves of light, yet 

 movement and interchange for all that. 



I mentioned red heat just now, and I did so because certain 

 geologists believe that this is the temperature to which granite has 

 been raised. This is not a guess. It is founded on the known 

 qualities of gases and steam. Their rate of expansion under heat 

 and pressure is calculated in connexion with the cavities in granite. 

 Some of these, it will be remembered, are half full of water, which 

 has been steam. The amount of condensation furnishes a factor 

 from which the former heat is estimated. 



Another kind of proof as to the origin of granite is found in the 

 sections of extinct volcanoes. A few instances are found which 

 enable us to see down into the innermost recesses of these subter- 

 ranean fiery lagoons. In the lowest depths where pressure of the 

 overlying lava prevented the escape of steam, the rock is granite. 

 In fact, the volcano itself is probably no more than the escape 

 through an accidental fissure of some of that heat which pressure 

 is causing below. 



"We must not, however, leave out of consideration one impor- 

 tant condition in these operations, and that is the length of time 

 through which they have been exercised. We have no standard 

 by which to measure it. The period of history occupies only a 

 few thousand years. Supposing the granite to have been seething 

 and baking amid steam at a red heat for that time, we can well 

 imagine surprising results. But probably nature's laboratory has 

 been working for cycles in which the historical period is only a 

 unit. What are the mutations observed in these granite hills as 

 a work for such eternal ages ? The silence and obscurity in their 

 history is one of those mysterious chasms to the edge of which 

 science has enabled us to climb, but where we can discern only a 

 depth which is unfathomable. 



But now to account for the presence of the tin in the granite, 



