OX CORDIERITE AS CONTACT MINERAL. ?>10 



has n cherry-blossom like structure, the cordierite-slate was known by 

 the name of Salura-islii, as in the case of the Tamba specimen. The 

 size of the crystal is variable ; usually about 2 cm. in length and 5 cm. 

 in breadth; but in some cases it attains the much larger dimensions 

 of some 5 cm. in length and 1*5 cm. in breadth. In specimens in which 

 the alteration has not much advanced, the fresh portion is colourless 

 and transparent, and the fracture presents a highly vitreous lustre 

 like that of quartz. 



The transverse section of the Cordierite crystal always presents an 

 hexagonal figure divided up into sectors (Fig. 1), the boundaries of 

 which are formed by the filling up of the black carbonaceous matter. 

 There is usually in the interior a central nucleus or core, also hexa- 

 gonal and parallel to the outer boundaries, but apparently devoid of 

 any radial divisions or sectors as in the outer. 



Under polarized light the different sectors are found to differ in 

 optical orientation ; the opposite [»airs, however, behaving alike. 

 Thus we see that the crystal must be made up of the union of three 

 individuals according to a twin law, forming a Trilling, which, as is 

 well known, often assumes a pseudo-hexagonal symmetry, when the 

 twinning plane is a prismatic face having an angle near to \20°. This 

 twin, which was first observed in the specimen from Bretagne by J)f.s 

 Cloizeaüx,* is very characteristic of Cordierite, and has also been des- 

 cribed from the Asama and the Laach crystals. j - But there are in 

 the case which we are now describing some points of peculiarity, 

 which have not been recognized in the others. These are, firstly, the 

 separation of the cross section into the outer and the inner portions, 



* Mantie] de Mineralogie. I. p. :C>.">. 



t HnssxK, (I.e.) and v. Las.vulx — Ueber Cordierifc-zwillinge in einetn Auswürflinge dos 

 Laaeher Sees— Zeitschft. f. Krystall. 1884. VIII, p. 76. 



