302 MESSES. G. A. J. COLE AND J. W. GREGORY 



Leaving the stream, then, as it breaks np into the numerous brooks 

 which drain the peat-bog at the head of the valley, we turn to the. 

 ridges on the east from which the material has been derived, in order 

 that we may see what light is thrown on the relations of the rocks 

 to one another. 



y. The Gabbro and associated Serpentines. 



At the summit of the Col du Gondran, along the ridge that 

 forms the watershed between the Durance and the Cerveyrette, the 

 igneous rocks are seen in situ. We here find the gabbro as a series 

 of rough crags forming the south-west spur of Le Chenaillet. The 

 rock is usually coarse-grained, the crystals being in places as much 

 as 60 millim. in diameter ; the texture is truly granitic. The rock 

 is a gabbro (euphotide of Prench and Italian authors), all the more 

 typical because its constituents, plagioclase and augite, have been 

 altered into " saussurite " and " smaragdite " respectively. Micro- 

 scopic examination shows that the rock has undergone considerable 

 change. In a less coarsely-grained specimen collected from near the 

 junction with a diabase-dyke, the felspar is often breceiated and 

 traversed by veins of secondary hornblende. The following analysis 

 by Delesse* shows that the felspar is labradorite : — 



Silica 49-73 



Alumina 29*65 



Protoxide of iron -85 



Oxide of manganese trace 



Lime 11-18 



Magnesia -56 



Soda 4-04 



Potash -24 



Water and Carbonic Acid 3-75 



100-00 



The magnesia is to be attributed to the serpentine-veins traversing 

 the felspar, and Delesse has also noted the presence of veins of cal- 

 cite, which may perceptibly increase the percentage of lime. 

 Boulanger f has also given an analysis of the felspar, and of what 

 he describes as a felspathic " pcite blanche " in the euphotide ; but 

 the results of this early analysis differ considerably from that of 

 Delesse, while the silica of the felspar is much higher than in those 

 occurring in typical gabbro. 



The pyroxene is still more altered. There is little of it remaining 

 even in the form of diallage, since it has been changed into grass- 

 green masses of " smaragdite ; " these on closer examination are 



* Delesse, " Eechercbes sur TEuphoticle," Bull. Soc. geol. France, ser. 2, 

 t. vi. p. 549. 



t C. Boulanger, " Memoire sur la Composition des rochcs d'cupbotide," 

 Ann. Mines, sur. 3, t. viii. (1835), p. 163. 



