﻿166 PEor. T. e, B02??fEY ox antigoritb [^ay 1908, 



into the former, and evidently are generally similar to some that 

 I have described from the Alps.^ Both, it is said, 



'have apparently resulted from the alteration of some olivine-roek, most 

 probably dunite, as judged by the presence of chromite in greater or lesser 

 quantity ' (p. 66). 



Some, however, of my specimens, as described in 1905,- exhibit very 

 clearly the direct production of antigorite from augite, and prove 

 that the latter mineral must have been so abundant in the original 

 rock as to make it an olivine-augite rock, like some of that in the 

 Yosges, where the pyroxenic constituent occasionally dominates.^ 

 It must also have been very closely related to the hornblendic 

 serpentines from the Lizard coast between llullion and Lower 

 Predanack,^ besides the well-known one from the Eauenthal^ (for 

 hornblende and augite are practically isomorphous), and in aU 

 these the monoclinic constituent is occasionally more abundant 

 than the olivine ® ; and that there was a like excess in some of 

 these specimens of Alpine serpentines appears, from the abundance 

 of the residual augite in the slices, to be very probable. 



That this mineral is converted into antigorite more readily than 

 olivine seems, however, fairly certain. AYhen I saw, now several 

 years ago, the bastite-serpeiitines in the Eastern Grisons and the 

 Mont Genevre district, I naturally took my specimens from the parts 

 least affected by pressure, and thus had hardly any material for 

 studying its effects. From this difficulty I was relieved by Miss 

 Eaisin, who very kindly halted on her way to Pontresina last 

 September to examine the serpentines near the Julier road, and has 

 provided me with an ample supply of specimens from the crushed 

 parts of outcrops east of Tiefenkastell, the Yal da Faller, and by the 

 Silser See.'^ These are seen at a glance to be very different from 

 the antigorite-serpentines. Pressure makes the latter fissile, con- 

 verting them, when it has been very severe, into slabs or even slates, 

 as already described, but it breaks up these bastite-serpentines into 

 lenticular masses, more or less irregular in form. Thfir exterior is 

 covered by a lustrous dark-green film, showing slickensides, but 

 when the rock is broken across its character is comparatively normal, 

 and flakes of bastite can sometimes be detected. 



I have examined slices from seven of the most promising speci- 

 mens. They come from an outcrop near the Silser See at the head 



' Geol. Mag. dec. iii, vol. vii (1890) p. 533, & dec. iv, vol. iv (1897) p. 110. 



- Quart. Journ. Geol. Soc. vol. Ixi (1905) pp. 706-12 ; also several specimens 

 collected since the reading of that paper. 



3 Op. cit. p. 697. 



•* This Tarietv occurs at other localities on the peninsula. See this Journal, 

 vol. Iii (1896) p. 46. 



5 Geol. Mag. dec. iii, vol. iv (1887) pp. 67-69, & Miss C, A. Eaisin, this 

 Journal, vol. liii (1897) p. 246. 



^ It is also noteworthy that the hornblende-serpentine at the Lizard is 

 distinctly harder to break and rougher to the touch than the ordinary bastite- 

 serpentine. 



' Described in my paper, Geol. Mag. dec. ii, vol. vii (1880) p. 538. 



