SURROUNDING THE LANd's-END MASS OF GRANITE. 411 



mineral which may be andalusite ; it gives moderately bright colours 

 in polarized light ; but as it only occurs in granular patches and 

 irregular plates closely aggregated, I have not been able to make a 

 satisfactory determination of it. 



St. Michael's Mount. — At the well-known junction of the granite 

 and slate on the south side of the Mount, a porphyrinic granite 

 occurs in contact with a fine-grained mica-schist of dark-purple 

 colour, the junction being very sharp and well defined. The granite 

 contains a few small prisms of tourmaline ; the mica is white, while 

 that in the altered slate is deep red. 



A thin slice showing both rocks contains a narrow vein extending 

 from the granite through the slate at right angles to the line of 

 junction; it is filled with white mica and a little quartz, and along 

 both sides numerous flakes of red mica are thickly crowded together, 

 being of larger size than in other parts of the slice. Another speci- 

 men from the Mount is a fine-grained white granite in contact with 

 a contorted mica-schist, the diminutive crumpled folds consisting of 

 red mica and small grains of magnetite. The crumpling was pro- 

 bably not caused by the intrusion of the granite, as the pressure by 

 which it was effected evidently acted at right angles to the line of 

 junction. This is the best example I have hitherto observed among 

 Cornish rocks of contortions on a microscopic scale. The structure, 

 however, is far more fully developed in other localities ; and I will 

 conclude this portion of the subject with a brief description of two 

 specimens selected from several, for which I am indebted to Prof. 

 Hull and Mr. Wylie ; they are from the granitic district of the south- 

 east of Ireland, where the granite is intrusive in Lower Silurian 

 slates. 



Plate XXIII. fig. 11 represents the metamorphosed portion of 

 a thin slice showing the junction of slate and granite near Ennis- 

 corthy, county Wexford. The granite is of medium texture, with 

 black mica disseminated through the mass. The altered slate is very 

 fine-grained and of a dark colour ; but with a lens there may be seen 

 numerous small specks of light colour, which impart to it an appear- 

 ance of foliation. 



The black mica of the granite is strongly dichroic, changing from 

 deep reddish brown to yellow ; the plates are often broken, ragged at 

 the ends, and the laminae partially separated from each other. The 

 altered slate is an extremely interesting object. It is an imperfectly 

 foliated gneiss containing numerous small well-formed crystals of 

 orthoclase surrounded by innumerable minute flakes of brown mica 

 which stream round them in flowing lines ; and these two constitu- 

 ents are enclosed in a quartzo-felspathie matrix. This specimen, 

 and others in my possession, show that some of the Irish Lower 

 Silurian slates are still more highly altered than those previously 

 described. Pig. 12 is a highly contorted mica-schist, also close to the 

 Wexford granite. The micaceous layers and alternating clear spaces 

 frequently contain Belonites, which always lie with their long axes 

 in the ever-varying planes of the contortions. I have omitted them 

 in the figure, as it is quite sufficiently complicated, 



