NOTES ON THE MINERALS OF JAPAN. 227 



Double twins have been observed in two pretty regularly 

 developed, dark-smoky quartz crystals from Mino, and in many 

 rock-crystals found near Kofu which are twins on P2. The 

 former tw^o specimens are Dauphine twins, which show Airy's 

 spirals in cross sections ; Fukuchi recognised in them, a com- 

 plicated distribution of electric poles after an experiment with 

 pyro-electricity. In the specimens taken near Kofu, there are 

 very often found Dauphine twins, again united on P2. 



In the Brazilian twins above enumerated, the twin-structure 

 was always observed in interference-figures and not in the 

 characteristic distribution of trapezohedral faces. 



Ordinary rock-crystals of Kai are sometimes more than 

 15 cm. in the diameter of basal sections ; but individuals 

 large enough to yield a perfectly clear colourless sphere of as 

 much as 13 cm. diameter, such as are sometimes to be found in 

 private collections, are no longer now to be had. 



Quartz crystals showing ±R, '^P, 4P, 2Pl', ePg, éPf, &c. 

 occui- in larger crystals at the localities 1, 2, 3, 6 and 7. 

 Many specimens from these localities show straight prismatic 

 edges and nearly equal -sized rhombohedral faces. Crystals with 

 rhombohedral terminations and curved prismatic faces are found 

 in the amethyst of Ohara, and in rock-crystals of Aikawa and 

 other metal mines. Trapezohedral faces are usually found on 

 larger individuals, sometimes however on smaller ones; they are 

 very seldom met with on llie small twins on P2 from Narushima. 

 Corrosion-faces were numerous on a naturally etched smoky quartz 

 from Kamikane in Kai, which will be treated of later on. 



Parallel growth is frequently found in the quartz of Tano- 

 kamiyama, where smoky quartz is often surrounded by a zone 

 of white or colourless quartz in the form of a complete cover. 



