430 ARKAN. MINERALS. 



I have not hitherto observed any marks to indicate 

 a passage into laumonite, ichthyophthalmite, or harmo- 

 tome. The analogy of the latter to the other zeolitic 

 minerals, appears indeed to be comparatively inconsider- 

 able ; but various specimens occur which seem to point to 

 a possible transition from the first of these substances 

 into stilbite, and from the second into analcime ; a mineral 

 presenting so many diversities of character as to require a 

 more accurate consideration than it has yet received. 

 The arrangement of the zeolitic substances appears indeed 

 still imperfect ; as I have already had occasion to notice in 

 speaking of the transitions which, in Sky, occur from 

 nadelstein to chert and to quartz, and of the great diversity 

 of chemical composition presented by analcime. 



But these transitions into the zeolites are not the only 

 examples of the varying character of prehnite. A distinct 

 transition into quartz is by no means rare in the tract 

 under review. In these specimens the centres of the 

 aggregated globules consist of prehnite, commonly of a 

 distinct fibrous texture. As the fibres extend from the 

 centre they become harder and lose their green hue, be- 

 coming also colourless, and at length transparent ; while 

 they enlarge in such a manner as to represent accumulated 

 prisms of quartz. The perfection of the change to quartz 

 is, in this instance, as in the change to analcime, put out 

 of doubt by the surfaces of the spherules being covered 

 with large and distinct pyramids of transparent quartz. 



I have not met, in this tract, with any decided examples 

 of a change to chalcedony, unless the condition which 

 takes place between the quartz and the prehnite may be 

 considered chalcedonic ; but such a change would excite 

 no surprise, when it is considered how slight the differences 

 are between that mineral and quartz. 



In attempting to apply geometrical analysis to these 

 varieties for the purpose of confirming the reality of those 

 changes, no additional light is thrown on the subject; 

 or at least no certainty can be procured respecting the 



