OX COKDIERITE AS CONTACT MINERAL. 333 



Explanation of Plate XXVIII. 



Fig. 1. — Transverse section of the Cordierite-twin, somewhat 

 idealized, showing the optic orientation. The directions of cleavage 

 are represented by line lines parallel to the outer sides (010) ; and of 

 interposition by thick lines at right angles to the sides, i. e., parallel 

 to (100.) 



Fig. 2. — Ideal form of the twin-crystal, consisting of the inner 

 and the outer portions, the boundaries of which are marked by the 

 shaded double-pyramid. 



Fig. 3. — Transverse section to show the peculiar structure of 

 the inner portion. 



Fig. ^.—Longitudinal section with characteristic diagonal 

 boundaries of the inner and the outer portions, and their directions 

 of growth. Natural size. 



Fig. 5. — The same, more perfectly developed, x 2. 



Fig. 6. — The same, but in a rudimentary or skeletal form, 

 consisting of the hue longitudinal bundles corresponding to the inner, 

 and the wedge-shaped cross bundles corresponding to the outer por- 

 tions in Figs. -1 and 5. x 3. 



Fig. 7. — Thin slide of the slate from Watarase-gawa with two 

 typical sections of Cordierite, the transverse (upper) and the longitu- 

 dinal (lower) sections, with characteristic interpositions, coloured yel- 

 low by the infiltration of the iron hydroxide (Limonite). Natural size. 



Fig. S. — Transverse section of three united individuals, probably 

 a twin-group, with a small transverse section to the left lower 

 side. Natural size. 



Fig. 9. — A part of the polished surface of the transverse section, 

 covered with numerous elongated corrosion-figures running parallel 

 to the sides (010). 



