383 



developed in contact-rocks. In addition to alumina it contains iron 

 and magnesia. It occurs as a contact mineral and also as a con- 

 stituent of certain crystalline schists. The small crystals in the spotted 

 mica-slate of the Vosges rarely measure more than < 02mm. in length. 

 They are distinguished from andalusite by the form of the cross 

 section and by the fact that the vertical axis is coincident with the 

 7 axis of elasticity. 



Sillimanite (Fibrolite). This mineral occurs in long and for the 

 most part extremely thin prisms. The prisms are separated at 

 intervals by cross joints. When they are not too thin they appear 

 colourless. Yery thin prisms may appear opaque in consequence of 

 total reflexion at their margins. As a rule the forms cannot be 

 determined in thin sections. The prismatic angle is about 97 and 

 the prism faces are often striated parallel to the long axis. 

 Refraction and double refraction are both higher than quartz. The 

 7 axis is coincident with the length of the crystal. Sillimanite occurs 

 in certain sandstones metamorphosed by contact with granite-masses. 

 Professor ROSENBUSCH also observed it in association with cordierite 

 in the cordierite-hornfels of the Vosges. It is also a constituent of 

 certain crystalline schists (e.g., fibrolite-gneiss). It is identical in 

 composition with chiastolite and andalusite. 



Dipyre. This is a mineral of the scapolite group. It has not been 

 recognized in this country but occurs, both in limestones and slates, 

 near granite-masses in the Pyrenees. It is colourless in thin section. 

 The refractive index of the mineral is about the same as that of 

 quartz, the double-refractive power is greater than that of quartz. 

 External crystalline form is wanting in the rock-forming members of 

 the scapolite group, except where they occur in limestones. All 

 members of the group are uniaxial and negative. 



Cordierite is rare as a contact mineral. It is found in the 

 Vosges and Mr. DILLER (I) records it as occurring under similar 

 conditions in the Troad. It is without form in the contact-rocks. 

 The most useful diagnostic characters are refraction, double-refraction 

 and the nature of the inclusions. The latter consist of fibrolite- or 

 sillimanite-needles and minute bodies round which there is a yellow 

 pleochroic border. 



Garnet is very commonly present in contact-rocks and is sometimes 

 so abundant as to make up the bulk of the mass. It occurs in 

 well- crystallized dodecahedra, irregular grains and compact masses 

 (allochroite). The variety known as grossularia is especially charac- 

 teristic of limestones when altered by contact with igneous rocks. 

 The red varieties (almandine) are more common in impure argillaceous 

 rocks. Allochroite is found in the St. Just district, on the west 

 side of the Land's End mass of granite. 



Mica. Both white and brown or black micas occur in the 

 contact-rocks. In the absence of tourmaline the micas generally 

 (1) Q.J.G.S., Vol. XXXTX. (1883), p. 627 



