28 GEOLOGICAL MEMOIRS. 



be a metamorphic limestone, is then particularly adverted to : its 

 characters are succinctly described ; and M. Delesse proceeds to say, 

 that probably the limestone was originally deposited either in mass 

 from water charged with carbonate of lime, or as strata by the waters 

 of the sea: the beds in which the limestone has been intercalated 

 belong without doubt to certain divisions of the Transition Group ; 

 and moreover all geologists who have studied the Vosges have re- 

 garded the gneiss enclosing the limestone as metamorphic. 



The pheenomena that have produced the metamorphism of the 

 gneiss are unknown ; but a group of strata could be transformed into 

 gneiss only by the introduction of the quantity of alkalies necessary 

 for the production of the felspar, one of the constituents of the gneiss. 

 Further, heat must have been effective in the development of the 

 crystalline structure of the limestone of the gneiss, since the lime- 

 stone contains spinelle, chondrodite, garnet, amphibole, pyroxene, 

 &c., that is to say, minerals of an igneous origin, since they are found 

 in the limestones on the flanks of Vesuvius, or in the sphere of action 

 of other volcanoes now active, such as those of TeneriflPe, Ponza Isles *. 

 On the other hand, there could not have been complete fusion, for 

 in the crystalline limestone of Norway MM. Naumann and Keilhau 

 have observed fragments of coralsf . 



The nature of the very numerous minerals of the crystalline lime- 

 stone also gives great improbability to the hypothesis of complete 

 fusion. It appears indeed that rocks which have been reduced to 

 a fluid state, and which have had an igneous origin, such as Lavas, 

 have always a very simple mineralogical composition. They are 

 essentially formed of two minerals ; the one, of the felspar class, in 

 which are concentred the alumine and the alkalies ; the other, of the 

 pyroxene or peridote kind, in which are concentred the oxide of iron, 

 magnesia, and lime. In "crystalline limestone," on the contrary, 

 there are various silicates, sometimes with a single base, some- 

 times with many ; and these silicates are often associated either with 

 free silex, or with silicates not saturated with bases ; moreover, 

 together with these silicates, there are very energetic, uncombined 

 bases, such as magnesia (periclase), alumine (corindon) ; there are 

 also metallic oxides, such as the oxides of iron, which, under certain 

 circumstances, appear to have been contemporary with the limestone ; 

 and there are compound oxides, such as the spinelles, perovskite, in 

 which the oxide playing the part of an acid (alumine, titanic acid) is 

 an acid much less energetic than the silex. We easily comprehend 

 then that these minerals have been formed with the concurrence of 

 heat, or of the molecular actions which it developed ; but it is difli- 

 cult to admit that they result from a complete fusion of the crystal- 

 line limestone. 



Moreover many facts prove that felspar may be formed in rocks 

 without the intervention of a great heat ; for example, in the arkose 

 of La Poirie (Vosges) crystals of felspar are developed in the clay- 



* Dufrenoy, Ann. des Mines, 3 ser. tome xi. p. 385. 



[t See also Translation of Prof. Scheerer's Memoir, supra, p. 7. — Ed.] 



