294 ■ EXPERIMENTS ON METAMORPHISM AND 



immense chasms.* This fact would be analogous to the condensation, 

 in the superficial beds of volcanic mountains and of lavas, of sal 

 ammoniac, the different chlorides, sulphur and specular iron, or to 

 the well-known enrichment of numerous metallic veins ia their upper 

 part. 



To sum up, when we are to explain the origin and the formation of 

 silicates in most of the rocks, it is not to the dr}^ but really to the 

 hydrothermal Avay that Ave must oftenest have recourse. This asser- 

 tion is founded on the following considerations: 



1. Formation by the wet way takes place at temperatures incom- 

 parably lower than the point of fusion; this is a condition of which 

 we have previously recognized the necessity. 



2. The hydrated silicates which we find in nature often associated 

 with anhydrous silicates are easily formed in the wet way, as we have 

 seen, at the same time as these last (zeolites with pyroxene, 

 chloritic schist with tourmaline and feldspar, &c.)- their forma- 

 tion is with difficulty explained in the dry way. 



3. Quartz is extremely abundant in nature. Now, as soon as super- 

 heated water is in contact with a great number of silicates, soluble 

 or insoluble, we see that a part of the silica isolates itself and becomes 

 a genuine crystalline quartz, which does not at all resemble the glass 

 produced by the fusion of quartz. 



It will be remembered in effect that silica, whether melted or 

 obtained by the decomposition of the silicates, has none of the prop- 

 erties of quartz; that it is not as dense, nor as refractive, nor as hard, 

 nor as refractory with alcaline agents. f It is possible that this dif- 

 ference of properties is the cause of the easy decomposition of the 

 vitreous silicates: the menstruum attacking the silica under its solu- 

 ble modification, then, perhaps, without there being any necessity 

 for a change of circumstances, they precipitate it under the modifica- 

 tion corresponding to insoluble quartz, only serving then, so to speak, 

 to cause the silica to pass by a kind of continuous evolution from one 

 molecular state to the opposite:}:. 



4. In fine, instead of uniform masses, such as fusion generally pro- 

 duces, we see in the products of the wet way mixtures of different 

 crystallized substances, whose mode of association is entirely inde- 

 pendent, as is the case with the greater part of rocks of their rela- 

 tive degrees of fusibility. 



'"■ This fact, which results from former and unedited observations of Elie de Beaumont, 

 has been recently pointed out also by 8ir Koderick Murchison. 



t Pogg. Annalen, 1859. 



J Stannic acid presents something similar, when we see one of its modifications, (stannic 

 acid, properly so called) passing by the simple action of heat to the other state of modifica- 

 tion, (metastanic acid,) and thus separating itself from certain solvents. 



