Quartz and Metalliferous Veins. 127 



loses its liquidity as it gradually cools, and, by its heat ex- 

 panding the rock, contracts the pores, and so itself bars the 

 passage. If besides this, as in the case in question, the 

 molten fluid is hotter than the fusing heat of the rock into 

 which it endeavours to penetrate, and if it have chemical af- 

 finity for its components, the consequence could not be an 

 impregnation but a fusion together. In the second place, a 

 watery solution, while it deposits matter in a rock, can dis- 

 solve others and carry them off; and if the contact with it 

 continues vei'y long, the exchange may be so considerable, 

 that great masses of the constituents may disappear, and just 

 as great masses take their places from the watery solution. 

 Such circumstances cannot be conceived as to the penetration 

 of molten substances : an exchange is impossible in this case. 

 A molten substance can only fill the pores of a rock, or only 

 enter in the proportion of these intervals. The expansion 

 of the stone, by contact with so heated a substance, may, 

 however, easily amount to more than the volume of these 

 pores, in which case the rock can only take up so much as 

 fuses on the surface of contact. 



In the granite altered by the quartz veins in the above- 

 named places, the felspar and mica are much diminished. 

 What, therefore, is more probable than that the Vv^atery 

 solution of quartz, which penetrated into tlie granite, gi^a- 

 dually decomposed and carried away these substances, pre- 

 cipitating instead its silicic acid 1 



Thus the double six-sided pyramidal crystals of quartz, 

 spoken of by v. Declien and v. (Eynhausen, may have been, 

 not the original quartz of the rock, but only that formed 

 from the watery solution. Where they observed the more 

 frequent occurrence of felspar, partly undecomposed, partly 

 weathered to porcelain-earth, the quartz was less prevalent. 

 In this instance, only the first stage of the decomposition of 

 the felspar is manifested ; but, in the entirely converted 

 rock, the porcelain earth had also been carried away, 

 quartz having taken its place. It is clear that such con- 

 siderable changes in a rock, by watery solutions, must have 

 required a very long period, especially as, in the case in 

 question, the mica, so difficult of decomposition, had dimi- 



