357 



a sufficient amount of individuality to enable their outlines and internal 

 structure to be made out. They show no trace of crystalline form, the 

 irregular grains fitting together so as to make up the matrix in which the 

 other constituents brown mica, apatite and magnetite are embedded. 



The mica occurs in thin plates, the edges of which are extremely 

 irregular, but with indications of the crystalline faces. The margin is 

 always darker than the central portion. The optic axial plane lies parallel 

 to the plane of symmetry and the plates are sometimes elongated in 

 this direction not regularly hexagonal. The more or less lath-shaped 

 sections often show under crossed nicols a lamellar twinning. Adjacent 

 lamellae extinguish in different positions ; the angle between the two 

 positions being about 6. This shows that the first mean line (negative 

 bisectrix) is inclined to the basal plane which is the face of composi- 

 tion. The lath-shaped sections show the usual dichroism. They 

 become colourless when viewed with rays vibrating at right angles 

 to the cleavage cracks. The plates of mica have frequently suffered 

 mechanical deformation. They sometimes contain lenticles of calcite. 

 Apatite is very abundant. It occurs in well-crystallized prisms with 

 pyramidal terminations and often shows cross jointing at irregular in- 

 tervals. Magnetite is present in well-crystallized octahedra. There are 

 some quartz grains which may be of secondary origin. Calcite-dust is 

 distributed in patches through the felspar, and there is also some calcite 

 which is definitely crystalline. The rock is a minette. It closely 

 resembles the Trelissick rock analyzed by Mr. PHILLIPS. Augite cannot 

 however, be recognized with certainty, although there are some green 

 patches which may represent this mineral. 



Other localities. Mica-traps occur in the Channel Islands. Two 

 of the Guernsey dykes are described by Professor BONNET in an 

 appendix to a paper by Mr. HILL. (I) That of Moulin Huet contains a 

 trachytic ground-mass mainly composed of elongated felspar microlites. 

 It is a kersantite-porphyrite. The rock of Bee du Nez is less markedly 

 trachytic in texture and is therefore more allied to kersantite. Allied 

 rocks are described by M. NounY (2 > from Jersey under the name of 

 micaceous porphyrite. 



The mica-traps of the Channel Islands doubtless belong to the 

 same period as those of Cornwall, Brittany, the Hartz and the 

 Fichtelgebirge. 



Mr. WARD (3 > describes a minette from Sale Fell, west of Bassenthwaite 

 Lake in Cumberland. Professor HuLL< 4 > states that mica-traps occur 

 as "dykes or beds amongst the Silurian rocks of Wicklow, Cavan, 

 Monaghan and Down; also in Co. Wicklow and other parts of Ireland, as 

 near Louisberg, Co. Mayo. In Scotland also they are found amongst 

 the Silurian rocks of the Southern Uplands, apparently continuous 

 with those of the North of Ireland." 



(1) Q.J.O.S., Vol. XL. (1884), p. 426. 



(2) Geologic de Jersey. Paris, 1886. 



(3) Survey memoir on the northern part of the Lake District, p. 33. 



(4) On Building and Ornamental stones. London, 1872, p. 83. 



