X20 X - YAMAS1KI. 



The most important and characteristic feature of this rock is the 

 presence of the piedmontite as the secondary product. This is general- 

 ly of large size, when found replocing the substance of the feldspar- 

 phenocrysts, but in drusy cavities forms tufts of fine needles. In the 

 former case, it does not take any regular form ; but commonly occurs 

 as a granular aggregate, or in an imperfectly developed columnar 

 shape, as shown in Fig. 2. A few quite perfect crystals were obtained 

 by cautiously breaking the aggregate in a cavity with the edge of a 

 knife. 



Some of them are shown in Fig. 3. As usual, they are acicular 

 in form, elongated in the direction of the 5-axis. Basal cleavage is 

 distinct, and some crystals are crossed by cracks transverse to the 

 direction of elongation. Parallel growth of the individual crystals is 

 not uncommon, and in consequence the surface of the crystals exhibits 

 fine striations. The predominating faces are (001), (100), (101), 

 (111). Besides, there are a few other faces, but so imperfectly de- 

 veloped that I could not measure them. Pleochroism is distinct, 

 ranging between nearly colourless, light-yellow, and rosy-red. When 

 the long side of the T (100) face coincides with the vibrating plane 

 of the lower Nicol, it is nearly colourless (slightly orange-yellow in 

 thick section), and when at right-angles it is light-rosy-red. M (001) 

 face of the crystal is nearly colourless and light-lemon-yellow in the 

 parallel and crossed positions respectively. Clinopinacoidal sections 

 show lemon-yellow to rosy-red. The axial colours, thus determined 

 are : 



<x= light-lemon-yellow, 



ß = colourless to light-orange-yellow, 



c=rosy-red. 



Absorption is c>a>ß. 

 The colours in general are exceedingly feeble in intensity as compared 



