292 EXPEEIIMENTS ON METAMORPHISM AND 



other origin. A tepid and scarcely mineralized water has been suf- 

 ficient to transform this masonry and produce in it hydrated and 

 crystallized silicates. Would not the efiect have been much more 

 considerable if water greatly superheated, and j^et retained by the 

 pressure of the masses above, had slowly circulated across certain 

 rocks, as it has done in the concrete of Plombieres, and had acted on 

 them at the high temperatures which are necessary to the formation 

 of anhydrous silicates ? 



CHAPTER VIII. 



OTHER PECULIARITIES OP METAMORPHISM EXPLAINED BY THE AID OF THE 

 FACTS MENTIONED IN CHAPTERS IV AND VI. 



In comparing* the results obtained by the experiments on super- 

 heated water, with the facts gleaned from the contemporaneous 

 phenomena of Plombieres, we can explain the greater part of the 

 })henomena of metamorphism. I shall add only a few examples to 

 those of which I have spoken above. One is the well-known de- 

 velopment of pyroxene and amphibole in the secondary limestones 

 of the Hebrides and the Pyrenees. I will mention also the production 

 of such various minerals in the blocks of limestone of Mount Somma, 

 tlie geodes of which are incrusted with diopside, mica, and other 

 substances. A frequent phenomenon of metamorphic rocks is the 

 development of feldspar in their mass. Among the numerous facts 

 of this kind, I will recall the schistose formations which border upon 

 granite, (Britanny, Saxony, &c.,) and even the schistose masses near 

 which we do not see any eruptive rock, (Taunus, Ardennes, <fec,) In 

 the carboniferous formations of the Vosges, atThann, for instance, the 

 perfectly regular beds of greywacke are studded with crystals of 

 feldspar, which have isolated themselves from a petrosilicious base ; 

 the numerous vegetable fossils which the rocks contain will not allow 

 of its being considered as a porphyry. The limestone of Mont Blanc, 

 formerly noticed by Brochant, and which Brongniart named cal- 

 ciphyre feklsjmthique, has peculiar analogies. We should not, how- 

 ever, lose sight of the fact that here, as in other cases of the same 

 kind, the limestone which has thus been modified has not always 

 changed its primitively compact for a crystalline state. 



Among the frequent associations of anhydrous and hydrous silicates. 

 I shall restrain myself to mentioning the chloritic rocks which form 

 the gangue of tourmaline, amphibole, pyroxene, &c. The cr3^stals 

 of the feldspar called adularia, which are penetrated with chlorite, 

 (Pfitschen, in Tyrol,) sometimes even with stilbite, (Sella, or St. 

 Gothard,) show us that the anhydrous were even sometimes crystal- 

 lized after the hydrous silicates.* 



~ Although chlorite has not yet been imitated, we may believe, from its chemical analo- 

 gies to the zeolites, as well as from the preference it shows for those formations which 

 have undergone a commencement of modification, as the schistose formations of the Ar- 

 dennes, that it has been formed at a high temperature. 



