296 EXPERIMENTS ON METAMORPHISM AND 



the surface, either in a state of vapor or as hot springs.* Let ns 

 remark, however, that this extravasement of minerals ready formed, 

 of which I spoke above, is, without doubt, only an appearance, and 

 that the feldspar or the mica which is in the neighborhood of granite 

 was probably formed in situ by borrowing, as at Plombieres, a part 

 of its elements from the medium in which it was developed. 



It is here that we may advert, in a few words, to the singular destiny 

 of pyroxene. The crystals of pyroxene, so frequently disseminated in 

 lavas, were formerly considered as having been detached from a pre- 

 existing rock, and, to better express the idea already put forth by 

 Dolomieu, that they Avere not formed in the volcanic rocks which 

 contain them, but that they had been simply imbedded in them. 

 Haiiy gave its name to pyroxene, in the sense which its derivation 

 indicates — stranger tofire.\ Afterwards it was ascertained that, on the 

 contrary, it had cr^'staJlized in the lavas, especially since the experi- 

 ments of Berthier and Mitscheralich, and it has been considered as 

 the type and exclusive product of the dry way. Is it not strange to 

 see that, by its great tendency to form in superheated Avater, it is this 

 mineral which appears at the present time the most important among 

 the best characterized products of this new method ? Let us add, 

 further, that Avater might have had its influence even in the mechan- 

 ical action of the eruptive rocks. Indeed, in the recital of my ex- 

 periments I have designedly dwelt on the augmentation of volume 

 w^hich glass transformed into a zeolite by the agency of water has 

 undergone, in order to draw from it the conclusion that, in all proba- 

 bility, at the moment of their hydration certain rocks have undergone 

 a phenomenon of increase analogous to that of Avhich Ave have numer- 

 ous natural examples Avhen anhydrite changes into gypsum. This 

 increase was probably sufficient in many cases to bring about the 

 extrusion and the eruption of rocks; this AA'ould be particularly the 

 case with phonolites and.basalts. 



CHAPTER XI. 



METAMORPHISM OF STRUCTUKE ; ITS RELATIONS TO ORDINARY METAMOR- 

 PHISM. 



"We have seen, in the first and second part of this memoir, that the 

 schistose structure appears to be the result of pressure and of slips 

 undergone by the strata under the action of energetic forces. The 

 experiments on this subject, AAdiich I undertook before knoAving of 

 those of Mr. Tyndall, but which I made by other processes and on a 

 larger scale, confirm this theory. I used for the purpose powerful 



•"■ To tbis are to be referred the springs of which metalliferous veins and other deposits 

 neighboring on eruptive rocks attest the existence. They have probably, with time, di- 

 minibhed in temperature and volume, and have at last dried up, when the masses from 

 which they came arrived at their last stage of consolidation and refrigeration. 



f " The name pyroxene apprises us that the crystals of lavas are not ia their native 

 place." — {Mineralogie de Zlai'iy, 1st edition, vol. iii, p. 90.) 



