366 



W. Cross — Some secondary minerals 



strong: a = green (bluish-green in thick sections), 6 = clear 

 green, c=yellow. Absorption: a >6 >c. Owing to the strong 

 absorption near a the position of this axis cannot be accurately 

 located, and the mineral seems to extinguish parallel to its ver- 

 tical axis. Sections cut normal to the prism show the optic 

 angle to be large, but the exits of the axes are much plainer 

 than in sections seemingly near to 00P00 , hence the mineral is 

 probably negative in optical character. 



The change from blue amphibole takes place with no inter- 

 mediate stage. It occurs mainly along the borders, and in less 

 degree from fissures. In either case the alteration proceeds 

 most rapidly from the starting point parallel to the vertical 

 axis, producing columns of the new substance. While ad- 

 ditions to the original amphibole grain do not appear, the 

 pyroxene substance often assumes it proper terminal form 

 when adjoined by calcite or quartz, and in case the latter 

 minerals have eaten into the side of the amphibole grain the 

 pyroxene substance may project in terminated crystal points 

 into the area of calcite or quartz, as illustrated by fig. 7, in 

 which the stippled area represents blue amphibole. The 

 boundary between the minerals is in this case a cloudy zone 

 and the amphibole is evidently giving way to calcite and 

 quartz. There seem to be pyramidal planes on the central 



crystal, but the others have rough faces. The section cuts 

 nearly parallel to ooPoo , as shown by pleochroism. The 

 needles or sharp and spine-like crystals of the same pyroxene 

 when found free in calcite have the forms shown in fig. 8. 

 Some of them seem to have pyramidal terminations with 

 distinct planes (a), while the majority are like crystals b, 

 and have apparently curved faces. Some crystals 0*4 mm long- 

 by 0'15 mm thick were isolated which are doubly terminated and 

 then show, when lying on a prism face, the monosymmetry 

 represented in fig. 8, c. This symmetry appears only where 

 the pleochroism indicates an approach to the position of the 

 clinopinacoid, and this fact is the strongest observable evidence 

 for referring the mineral to the monosymmetric system. Ob- 

 lique extinction in such sections cannot always be made out. 



