on Serpentines in the Pennine Alps. 541 



a schisty serpentine into a talc-schist, either much Si0 2 must be added 

 or much MgO removed. The condition and structure of the rock 

 is not favourable to the former explanation ; it seems more probable 

 that the alteration has been by a ' leaching out ' of the magnesia. 

 It is therefore difficult to understand the reason of such different 

 changes in two adjacent rocks, unless it be that an aluminous silicate 

 more readily parts with its silica, and a magnesian silicate with its 

 magnesia, in favour of which there is some evidence. At present, 

 however, I think it safer to restrict myself to calling attention to 

 the facts. 



Other specimens of slaty serpentine in my collection exhibit a 

 structure like that described above. For instance in one from the 

 western side of the Col di Vallante (Cottian Alps), which is extremely 

 fissile and of a distinctly green colour, the microscopic structure is 

 yet more minute, the rock more fissile (for it splits up in grinding), 

 but the slice gives higher polarization tints. This also exhibits an 

 occasional bending or ' rucking ' of the ' foliation,' which sometimes 

 has produced a ' strain-slip ' cleavage. Another specimen, from 

 near Verrex (at the opening of the Val d'Ayas), exhibits a like 

 structure, but in it, I think, a little enstatite has been present. ]n 

 one less conspicuously fissile, from the junction of glens at the head 

 of the Val Malenco, enstatite and augite are certainly present. 

 Sometimes the latter mineral is crushed to a powder, which has 

 a rather dusty aspect, while the larger granules exhibit the bright 

 tints customary in augite. Here and there a characteristic fragment 

 of that mineral can be found, and we may note that, though the 

 pressure has been very great, it remains augite, and has not been 

 altered into hornblende. 1 This variety of serpentine must exist in 

 some part of the massif in the region of the Gorner glacier, for I 

 collected several years since a specimen on one of its moraines, 

 of which, as I had some doubt as to the true nature of the rock, 

 I examined a slide. Here the matrix is less completely crushed 

 than in the specimens described above, and there is a large amount 

 of this granular pulverized augite (at least I identify it with this 

 mineral after comparison with the better pi-eserved examples in the 

 last-named slide). Specimens from other parts of the Alps exhibit 

 both varieties of serpentine, and the effects of less severe pressure ; 

 but on these it is needless to enlarge. 



It has been suggested that the streaky structure which is rather 

 conspicuous in some of the Lizard serpentines (e.g. a variety on 

 Goonhilly Downs and a common type at Porthalla 2 ) may be due to 

 pressure. 3 Applying the knowledge obtained in the present 

 investigation, I may venture to express the opinion that these Cornish 

 rocks have not been materially affected by pressure since they 



there can be little doubt the rock is an altered serpentine. The dominant mineral has 

 all the characters of talc, but there are several grains of calcite "which, by their mode 

 of occurrence, suggest replacement (? of augite). 



1 There are however not a few minute grains of a honey-brown, somewhat dichroic, 

 mineral which may be a variety of hornblende. 



2 Q.J.G.S. vol. xxxix. pp. 21-23. 



3 Geol. Mag. Dec. III. Vol. IV. p. 137. 



