ON THE FORMATION OF CRYSTALLINE ROCKS. 285 



dony. As the glass Avas as yet only altered on its surface, this deposit 

 must have corae, almost all of it at least, from the decomposition of 

 the alkaline silicate contained in the water of Plorabieres. Thus, 

 without the use of a single chemical reagent, under the single influ- 

 ence of heat, water holding alkaline silicates, such as those of the 

 springs of Plombieres in solution, deposits crystallized or crystalline 

 quarts. 



A new proof of the facility Avith which minerals of the feldspar 

 group can be produced in the presence of water is furnished by the 

 following experiment, which I made with a view of explaining the 

 feldspathization of many rocks, even those containing fossils. Kaolin, 

 perfectly purified from all feldspathic debris, by washing, having 

 been treated in a tube with water from Plombieres, this earthy mass 

 was transformed into a solid substance, confusedly crystallized in little 

 prisms which scratched glass. After having purified this substance 

 by washing it with hot water, we find that it has become fusible into 

 a white enamel; chlorhydric acid no longer attacks it. It is a double 

 silicate of alumina and an alkali, having all the characters of a feld- 

 spar; it is mixed with a little crystallized quartz. The reaction 

 which we have just explained may be compared to the ease with 

 which a cold silicate of alumina absorbs lime in hydraulic mortar. 

 On the surface and in the interior of the whitish mass resulting from 

 the transformation of the tube I found a great quantity of very small 

 crystals, but of a perfectly distinct form, having great brilliancy and 

 perfectly transparent; they had different tints of green, and many of 

 them that olive-green tint which is peculiar to peridot. Their form 

 is that of an oblique symmetric prism, the bases of which are re- 

 placed by two bevels; the two opposite edges are ordinarily truncated, 

 as in the pyroxene which Haiiy called homonome. These crystals 

 scratch glass very perceptibly; they remain unaltered in the presence 

 of concentrated and boiling chlorhydric acid. They melt before the 

 blow pipe into a black enamel. In fine, they have the composition of 

 a pyroxene, having a base of lime and iron; and from their transpar- 

 ency, they belong to the variety diopside. Some of these crystals are 

 isolated, some grouped together in such a way as to form little glob- 

 ules bristling with points, and more rarely they are under the form 

 of thin incrustations. All of them, by their aspect, immediately recall 

 the best known crystals of diopside. 



The clay of Klingenberg, near Cologne, which is used to make glass- 

 house pots, when heated in glass tubes, is covered with a multitude 

 of white, pearly pellicles, possessing the lustre of mica. They are 

 hexagonal, and have an optical axis of double refraction.* They are 

 fusible, and indicate before the blow-pipe the presence of silica. 

 They are attacked by chlorhydric acid, which manifests the reactions 

 of alumina. The quantity of these pellicules that I have obtained has 

 been as yet too small to permit of my making a quantitative analysis. 

 It appears, however, very probable that the substance is a mica of 

 one axis or a chlorite. 



'According to an examinatioa which M. de S^naimont was kind enough to make. 



