280 EXPERIMENTS ON METAMOKPHISM AND 



rine and boron,* I long ago showed that they appear to have been 

 instrumental in the formation of masses of tin.f In effect they enter 

 into the composition of characteristic silicates, as topaz and tourma- 

 line, which were certainly formed at the same time as the oxide of 

 tin.t 



These conclusions are equally applicable to the rocks whose origin 

 is due, according to all probability, to analogous phenomena. Such 

 is the well known rock found at Schneckenstein, in Saxony, where 

 topaz and tourmaline appear to have glided between the divisions of 

 the schists, at the same time cementing, in conjunction with quartz, 

 the numerous fragments into which this schist had been broken. The 

 same is true of entire formations in Brazil, such as those that contain 

 topaz in the county of Villarica — schists in which gold and the diamond 

 with the same characteristic minerals have been found over vast ex- 

 tents of country. These formations are, as it were, but an accumula- 

 tion over a great space of the habitual gangues of oxide of tin.§ 

 There still remains in granite a sufficiently sensible quantity of fluo- 

 rine and of boron to allow us to admit that it is possible that this 

 rock, before solidifying, gave off large quantities of vapors in which 

 those two bodies were in combination. These ideas on the interven- 

 tion of fluorine and boron, which date twenty years back, have ac- 

 quired still greater value since Henry Deville has crystallized a 

 series of minerals by the aid of the fluorides, and since the presence 

 of fluorine and boron has been discovered in many mineral waters, and 

 fluorine even in sea-water. 



We can explain by heat, accompanied by the auxiliaries which 

 have just been mentioned, a greater number of transformations than 

 by heat alone; but with those agents only we cannot explain some 

 very important circumstances, except by attributing to the vapors a 

 part evidently much exaggerated. Bischof, and other savans have 

 clearly substantiated this fact by numerous considerations. |1 « 



* The presence of fluor, already discovered in different modern volcanic rocks, has been 

 verified by M. Scacchi in a recent deposit of the fumaroles of Vesuvius. With regard to 

 boron, the enormous quantities which come from the saffioni of Tuscany and the extensive 

 deposits of the craters of volcanoes hardly permit us to doubt that it exists in many other 

 localities which have been passed unnoticed up to the present time. 



tOn the origin and artificial production of oxide of tin. — i^Annales des Mines, 3d series, 

 vol XX, p. 65, 1841 ; 4th series, vol. xvii, p. 129, 1849 ) 



J This first comparison between boron and tin, established on purely geological grounds, 

 has been followed by the discovery of an unexpected analogy between two bodies whose 

 chemical properties are so different. I speak of their isomorpliism, which has been demon- 

 strated by the labors of M. Sella. 



. § I am very far from thinking that these different quartzose rocks have been formed 

 •without the presence of water. I shall presently revert to this subject. 



II How, for Instance, can we admit of sucii an origin for the formation of crystals of 

 feldspar or garnet in regular strata, which are often hardly modified at all ? 



