of Mineral Species. 107 



other, testifying the progress of the change ; and we must be 

 the more careful in establishing hypotheses, if, as in the present 

 case, we are not led by analogous occurrences in other varieties 

 of the same species. 



Calcareous spar is one of those species which are very easily 

 acted upon by atmospheric agents. The hollow scalene six- 

 sided pyramids of brown-spar, the macrotypous lime-haloide of 

 MOHS, consisting of imbricated rhombohedrons with parallel axes, 

 form a remarkable instance in this species of the replacement of 

 one substance by another, not sufficiently explained by any of 

 the authors which treat of it, though some of the observations on 

 which the actual explanation of the appearances is founded, may 

 be traced in several of their writings. A specimen of a pale yel- 

 lowish-grey colour in Mr ALLAN'S cabinet, of the nature alluded 

 to above, and broken across, in order to shew the inside, presents 

 a cavity, the sides of which are lined with small rhombohedrons 

 of brown-spar, forming a surface analogous to the external one 

 of the six-sided pyramid. But it shews, besides, also the remains 

 of what formerly filled up the space altogether, of a crystal of the 

 rhombohedral lime-haloide. The planes of cleavage of this crys- 

 tal are still visibly in the same position in which they originally 

 existed, as appears from the contemporaneous reflection of the 

 image of a luminous object from the portions of it, now no longer 

 cohering. The surface of these portions has the same appear- 

 ance as fragments of calcareous spar which have been exposed to 

 the corroding action of acids. Crystals of the brown-spar are 

 likewise deposited on some of those portions disengaged from 

 the rest, and, as it were, pushed off from their original position, 

 by the gradual increase of the crystals of brown-spar. The mass 

 of this latter species forms a coating of pretty uniform thick- 

 ness over the whole surface of the original six-sided pyramid. 

 Nearly in the middle of the stratum, wherever it is broken across, 

 may be observed a whitish, or only rather more opake line, of 



