428 ARRAN. MINERALS. 



case they might easily be mistaken for the globular 

 varieties of siliceous schist occurring in the Shiant isles 

 and elsewhere, In the last modification of colour which 

 it is necessary to describe, the tints are disposed, as 

 in natrolite, in zones parallel to the centre of the nodule ; 

 the different hues being white, green, and purple, forming 

 a very ornamental mineral, and presenting yet another 

 point of resemblance between this substance and a com- 

 mon member of the zeolite family. 



The fibrous variety presents considerable diversity of 

 aspect, all consisting in the greater or less perfection 

 of the texture, as it proceeds in gradation from the solid 

 modification, or vacillates towards the laminar. During 

 this progress, its transparency also increases, being most 

 perfect in those specimens which possess the fibrous tex- 

 ture in greatest perfection. In these cases it sometimes 

 retains the usual green colour, but occasionally also be- 

 comes colourless ; and thus some of the varieties are 

 found to be glassy, fibrous, and transparent. In examining 

 the botryoidal surfaces in these specimens, they present 

 the terminations of square prisms, sometimes truncated 

 at right angles by a surface either flat or convex; at 

 others, bevelled on two opposed faces, also by flat or 

 by curved surfaces, so as to form wedges, but generally 

 too minute and irregular to admit of being easily defined. 

 Occasionally however, the prisms are elongated and more 

 perfect, although, even in these cases, they are accumulated 

 into groups. 



This variety therefore undergoes a gradual set of 

 changes, in texture, colour, and transparency, by which 

 it approximates to nadelstein. It is true that I have 

 not in any instance discovered a complete and final 

 transition into that mineral; but the interval remaining 

 unfilled is not considerable, while, in many specimens, the 

 nadelstein is associated with the prehnite in such a state 

 of indiscriminate mixture, that the two cannot be se- 

 parated. 



