32 Formation of Quartz Crystals, S^c.from Siliceous Solutions. 



to the touch, and when it has imbibed water, becomes suffi- 

 ciently tenacious to be kneaded between the fingers. I can- 

 not believe that this matter is owing to a decomposition of 

 opal, similar to that which we have just mentioned, since, 

 from the manner in which it occurs inclosed in the rocks, it 

 could not have been exposed to the influence of the air. I 

 am rather of opinion that it is a particular state of opal. The 

 workmen also distinguish those earthy parts, which they re- 

 gard as opal that is not yet ripe, from those which are pro- 

 duced by the exposure of opal to the air, which they name 

 burnt or calcined opal. These matters harden a little on ex- 

 posure to the air, and crack in collections, precisely the same 

 way as alumina or silica in a state of jelly, which are desic- 

 cated in our laboratories. It has been without doubt obser- 

 vations of this kind, which have led certain authors to say, 

 that opals are found, when in the bowels of the earth, so soft 

 as to receive the impression of the fingers, and that they har- 

 den only by exposure to the air. This idea is not perhaps so 

 ridiculous as might at first be imagined ; for we know that si- 

 lica in solution assumes in drying, a certain degree of hard- 

 ness, and a lustre approaching to that of opal. It is true that 

 the greater number of opals are solid when taken from the 

 rock; but after finding them occasionally still soft, and capable 

 of drying in the air, might it not be supposed, that the rest 

 have undergone this desiccation in a slower manner in the 

 bowels of the earth ? By admitting this hypothesis, we can 

 discover the reason of the difference which exists between the 

 hyaline quartz and opal ; the quartz will be the product of a 

 crystallization of the siliceous matter, and the opal the result 

 of the desiccation of a gelatinous precipitate. I must remark, 

 however, that this is merely a hypothesis, which, while there 

 are some facts in favour of it, has also others against it ; such 

 for example is the existence of opal stalactites, with regard to 

 which it must be admitted that the matter has been in a kind 

 of solution." 



