An " Ovenstone" from Zinal, Canton Valais. 115 



The specimen which shows considerable signs of crnshing has 

 a foliated structure somewhat resembling the above described, but 

 rather less conspicuous. It also contains a greenish mineral, which 

 occurs both in irregular streaks and in mottled patches, down to 

 mere specks ; this, however, has a weaker pleochroism, but high 

 polai;ization tints and straight extinction, and is no doubt antigorite. 

 There is some iron-oxide, but the specimen differs little from other 

 crushed Alpine serpentines which I have already described.^ The 

 specimen of massive aspect is, macroscopically, a dark-green rock, 

 like the last, but is mottled with spots of paler and grej'er green. 

 The ground-mass, on microscopic examination, consists of a minute 

 speckled mass of ordinary serpentine, in which flakelets of antigorite 

 occur like an irregular stitching. The lighter spots prove to be 

 a slightly brown augite, often " dusty " in aspect, owing to a minute 

 granular structure. These occasionally enclose grains of serpentine, 

 indicating a slight "lustre-mottling." The edges of the augite are 

 sometimes very slightly frayed out, but there is nothing to connect 

 it with the formation of antigorite. Magnetite is fairly abundant, in 

 grains, occasionally showing some faces of a crystal and in granules, 

 both with a clustered habit. This rock evidently belongs to the group 

 of Alpine serpentines which has been derived from augite-peridotites.- 



The green-schist, though also affected by pressure, is a very 

 different rock from the " ovenstone." Macroscopic examination 

 alone shows it to be a very characteristic, rather compact member of 

 the green-schist group (Griiner Schiefer). Under the mici'oscope it 

 exhibits a slightly banded, foliated structure. There are two pre- 

 dominant minerals : one is a chlorite, which occurs both in separate 

 flakelets and in streaky aggregated flakes, with pleochroism from pale 

 ochre to dullish green, and oblique extinction, the angle in some 

 cases being quite 25° ; the other, which occurs in grains and 

 granules, with rather irregular outlines, suggestive of a clastic (no 

 doubt cataclastic) origin, is water-clear, with rather high polarization 

 tints, one or two grains being twinned apparently on the Carlsbad 

 type. It encloses often minute flakes of chlorite and almost colourless 

 belonites (? actinolite). Probably it is a secondary felspar.^ A fair 

 amount of calcite is found, and granules of a slightly yellow tinge 

 are rather numerous, especially with the chlorite. I think most of 

 them are epidote, though sphene also may be present. Except for 

 one or two scales of hematite, iron-oxides are absent. That the rock 

 is one of the Alpine " green-schist " group, is beyond all doubt.* 



A study of this " ovenstone " confirms the view which I have 

 already expressed that a talcose schist may be only a further stage 



1 Some of it shows strain phenomena, indicating the action of a pressure subsequent 

 to that which has produced the general structure of the rock. 



^ Loc. cit., p. 462. 



^ This is the mineral which in some earlier papers I suggested might be a variety 

 of kyanite. It is a frequent constituent of these green-schists. 



* In these rocks, as I have already described, the green mineral is sometimes 

 a prismatic or acicular hornblende, sometimes chlorite, and both minerals may be 

 present. The " chlorite schist" of the Start and Bolt Head district in South Devou 

 is a rock of similar character to these Alpine green-schists. 



