308 MESSRS. G. A. J. COLE AND J. W. GREGORY 



segregation- veins, and the irregular nature of the junction and the 

 indusions of the plagioclase- and pyroxene-fragments from the 

 gabbro speak unmistakably in favour of their intrusive nature. 

 The rarity of the glass-selvage products is of interest, since it shows 

 that even the small intrusive veins could only have solidified slowly ; 

 the gabbro, therefore, may have been in a heated condition at the 

 time of the intrusion of the more compact rocks. The fact that the 

 dykes do not run from the gabbro into the diabase, suggests that 

 the former was more deeply seated at the time of their intrusion, 

 and that it was subsequently forced up into the diabase-masses by 

 earth-movements ; if, indeed, these dykes entered the diabase-masses, 

 the original junctions have been shifted and destroyed. 



Dykes, however, do occur in the great variolitic diabases. They 

 may be divided into three classes : — compact diabase ; diabase- 

 porphyrite or altered augite-andesite ; and coarse-grained dolerite. 



Commencing with the dykes of diabase, the most important is 

 one that crosses the ridge of Mt. La Plane, just south of the summit 

 and on the north side of the east hollow ; it is probably continuous 

 with a similar dyke seen in the corresponding position on the other 

 side of the ridge in the lower part of the east Chenaillet valley. 

 The rock is just macrocrystalline, and is jointed into bold columns 

 which are not perpendicular to the edge of the dyke. Microscopic 

 examination shows that the rock is composed of augite and plagio- 

 clase grouped ophitically ; the augite is remarkably fresh, but the 

 plagioclase is kaolinized. Patches of yellowish-green serpentine 

 indicate by their structure that they have been derived from olivine. 

 Biotite occurs as a few small, pale green, fibrous, and slightly 

 dichroic areas. There is a good deal of titanic iron with leucoxene. 

 The rock may therefore be called an olivine-diabase. It is markedly 

 scoriaceous, and overlies a thin diabase-ash or mud, which is baked 

 at the junction, and jointed into irregularly columnar blocks. This 

 green to purple slate-like rock appears to be unique in the area 

 under discussion. The dyke has cooled against the ash witli a vario- 

 litic selvage, a fact important in showing the connexion between the 

 diabase-dykes and the variolitic series. Immediately to the north 

 of this is a similar but less interesting dyke in the north-east hollow 

 of Mt. La Plane. A. dyke of a similar but more finely crystalline 

 rock runs up into the porphyritic diabase or diabase-porphyrite on 

 the west slope of the middle of the Chenaillet ridge. 



The " diabase-porphyrite " plays an important part in the forma- 

 tion of the Chenaillet ridge, which it crosses as a saddle-shaped mass 

 running north-east and south-west. It faces the west as a steep 

 and rugged crag, while it presents to the east a slope covered with 

 spheroidal joint-surfaces. Along its northern margin it cuts sharply 

 against a bank of tuff, but its relations to the diabase and tuff on 

 the west flank are complicated. In fig. 3 the junction of the 

 three rocks is shown ; the diabase-porphyrite is exposed by the 

 denudation of the diabase and tuff into which it has been faulted. 

 It appears here and at other points that the porphyritic rock was 

 intrusive into and partly faulted up through the diabase-series, of 



