ARRAN. MINERALS. 427 



The colours are various, even in the different modifications ; 

 but tints of green are prevalent in all those which possess 

 an ordinary or a medium character ; while those which 

 are at the extreme of either of the three textures, are 

 commonly either white or colourless, according to cir- 

 cumstances that will immediately be stated. The green 

 colour varies from the palest tints of yellow, ordinary, 

 and sea green, to shades of considerable intensity; yet 

 never attains a hue so strong and bright as that of the 

 prehnite found at the Cape of Good Hope. Occasionally, 

 it is evidently coloured by copper, although the general 

 green tint appears to be independent of that substance. 



In all these cases the prehnite continues to be more 

 or less transparent, -as it is also found to be when pos- 

 sessed even of a sap-green colour. But a variety, of a 

 dark bottle green hue, also occurs, which, a*s it becomes 

 darker, also becomes opake, and is generally intermixed 

 with talc of the same tint. 



Among the more transparent varieties, pale yellow 

 brown is occasionally found, as well as pale grey; this 

 latter modification, as it gradually loses its colour, be- 

 coming so transparent as to show the hue of the 

 rock to which it is attached : in this case also the frac- 

 ture resembles that of chalcedonic quartz, presenting 

 no apparent internal texture. Purple brown passing to 

 a more reddish purple is not uncommon in the semi- 

 transparent varieties ; and the colour sometimes gra- 

 dually vanishes in the same specimen by an apparent 

 dilution. The former colour is occasionally so intense as 

 to produce a perfect opacity ; and the specimens of this 

 description cannot then be distinguished from the brown 

 rock with which they are associated, except by a fresh 

 fracture. In this however, it is easy to perceive the 

 radiating structure of the globules which form the mass, 

 which is commonly fibrous with a slight tendency to the 

 lamellar, and either opake or glistening. These speci- 

 mens also present at times a botryoidal surface ; in which 



