90 Mr Haidinger mi the Parasitic Formation of Minerals^ 



is no proof, liowever, that such a process has actually taken 

 place, so long as we do not discover the remains of the former 

 species included in the other, testifying the progress of the 

 change; and we must be the more careful in establishing hy- 

 potheses, if, as in the present case, we are not led by analo- 

 gous 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 replace- 

 ment of one substance by another, not sufficiently explained 

 by any of the authors who treat of it, though some of the 

 observations on which the actual explanation of the appear- 

 ances is founded, may be traced in several of their writings. 

 A specimen of a pale yellowish-gray colour in Mr Allan's ca- 

 binet, of the nature alluded to above, and broken across, in 

 order to show 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 shows, 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 crystal 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 appearance as fragments 

 of calcareous spar which have been exposed to the corroding 

 action of acids. Crystals of the brown-spar are likewise de- 

 posited 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 thickness 

 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 opaque 

 line, of the same colour as the rest, dividing it into two, with- 

 out producing the least deviation in the faces of cleavage up- 

 on which it is seen. This line is evidently the section of the 



