284 EXPERIMENTS ON METAMORPHISM AND 



pyramidal form of quartz, and which, in reality, is nothing more than 

 crystallized quartz. Some crystals, thus formed, have attained at the 

 end of a month .07 of an inch in length. Sometimes they are iso- 

 lated in the opaque paste, sometimes lodged on the sides of the glass 

 tube, where they form veritable geodes, which, with the exception of 

 their size, it would be impossible to distinguish from those so fre- 

 quently found in crystalline rocks. What renders this transformation 

 of glass still more remarkable in a geological as well as a chemical 

 point of view is, that it is obtained with a very small quantity of 

 water, which in weight is not equal to a third of that of the trans- 

 formed glass. 



Volcanic glass, known under the name of obsidian, acts in a manner 

 similar to the artificial. Pieces of obsidian, heated under the same 

 conditions, are changed into a gray product of a crystalline nature, 

 having the aspect of a fine-grained trachyte. Its powder, examined 

 under the microscope, presents exactly the characters of crystalline 

 feldspar, and especially resembles rhyacolite or vitreous feldspar. In 

 fact, we know that obsidian greatly resembles feldspar in its chemical 

 composition; favorable circumstances would, without doubt, have de- 

 termined the reunion of its elements in definite proportions. The 

 tendency of forming in the wet way that feldspar thus manifests is 

 to be taken into account in divers geological circumstances. 



With the fragments of obsidian on which I operated there were 

 pieces of vitreous feldspar detached from a trachyte of Drachenfels, 

 and, also, a piece of oligloclase from Sweden. These two last min- 

 erals underwent no appreciable change.* We cannot, however, af- 

 firm that if the water had not immediately found an alkali to remove 

 from the vitreous envelope it would not have acted upon the feldspar. 

 We here see a kind of a confirmation of the preceding experiment on 

 the stability of the silicates, which have, perhaps, originally crystal- 

 lized in conditions very similar to those in which they were again 

 placed. The same is very nearly true of thin laminae of the potash 

 mica of Siberia; they hardly lost their transparency. Crystals of 

 pyroxene also do not change their aspect, except that, like the pieces 

 of feldspar and obsidian, they are so completely enveloped with 

 crystals of quartz that it is necessary to break in order to recognize 

 them. 



To examine, as far at least as the presence of glass will permit, 

 how the solutions of natural silicates which we usually find in water 

 act when superheated, I used water from the hot springs of Plom- 

 bieres, which is comparatively rich in silicates of potash and soda. 



Not being able, however, to operate on more than from twenty to 

 thirty cubic centimetres, I concentrated it beforehand by an evapora- 

 tion so rapid that the carbonic acid of the air did not sensibly decom- 

 pose the silicates, and in such a way as to reduce it to one-twentieth of 

 its former volume. After an experiment that was stopped at the end 

 of only two days the sides of the tube were already covered with a 

 silicious coating in the form of crystallized quartz, and also of chalce- 



*^'Feldspar, however, can be decomposed cold by trituration. — {Annates des Mines, 5th 

 series, vol. xii, p. 547 ) 



