On Mineral and Metallic Veins. 



485 



Quartz also, in many instances ; and we can witness the growth 

 of calcareous spar as it gradually accretes from the roofs of cal- 

 careous caverns. We cannot imitate the forms, it is true, of 

 many of the crystallized substances from mineral solutions ; but 

 it is because we are unable to conduct experiments which, per- 

 haps, require thousands of ages of uninterrupted process. 



It may be considered as giving weight to the igneous theory, 

 that with few exceptions, the metals are found in the inferior 

 rocks. In the immense deposits that lie above the coal beds 

 we find, with the exception of ores of iron and a little copper 

 slate, no segregation of metals into veins or masses resembling those 

 in the inferior rocks. We find oxydes of iron in indurated masses, 

 and in the state of bog ore, but these are clearly a rifaccimento 

 from metals produced originally from the more ancient beds. 



Perhaps, also, it is not true that all the metallic substances 

 which have been brought into fissures from below, have arrived 

 there in a molten state. Many of the substances found in them 

 are capable of solution in hydrogen gas. Silver will deposit it- 

 self on substances suspended from the roofs of mines. Acicular 

 crystals of lead are often found adhering to the walls of mines 

 that have been closed a long time. The formation of saline 

 matter on walls, and the spontaneous production of nitre on 

 limestone, show that we are not yet acquainted with the princi- 

 ples upon which this branch of crystallization depends. We had 

 occasion personally to observe, whilst on a visit to Mount St. 

 Michael's in Cornwall, that on the sides of some masses of granite 

 that had been separated from the rock for a great period of time, 

 but which had evidently never been the wall of a fissure or vein, 

 that amidst a profusion of small crystals of quartz, several hun- 

 dreds of small white topazes were apparently forming. We 

 were struck with the circumstance, and Sir John St. Aubyn 

 kindly permitted us to bring some specimens away, together with 

 any mineral substances we thought worthy our attention. The 

 rocks there contain very curious minerals, but they are all con- 

 tained, as the fine blue crystals of apatite are, in small veins. 

 The topazes, on the contrary, were spread indiscriminately over 

 the surface of immense fragments of granite, anciently separated 

 from the mountain mass. 



It would also appear as if some crystallized substances are the 

 effect of a chemico-electric action between the wall, to which 



