depending on their Internal Changes. 91 



original surface of the pyramid of calcareous spar, upon which 

 one portion of the brown-spar was deposited, while another 

 portion was formed within the space previously occupied by 

 the calcareous spar, and destroyed in the progress of decom- 

 position. The chemical change is here very distinctly indi- 

 cated ; part of the carbonate of lime is replaced by carbonate 

 of magnesia, so as to form in the new species a compound of 

 one atom of each. How this chans^e was^ brouo^ht about is 



3^ ""'" «xw^.^. 



a 



difficult question to resolve, though the fact cannot be doubt- 

 ed, as we have, in the specimen described, a demonstration of 

 it, approaching in certainty almost to ocular evidence. It is 

 scarcely surprising that such appearances should be visible in 

 metallic veins, like some of those near Schemnitz in Hungary, 

 the whole nature of which shows that they must have been 

 gradually changed by successive revolutions, the uppermost 

 part being often almost entirely composed of cellular quartz, 

 which is formed in fissures contained in other species or com- 

 pound masses, subsequently decomposed, and leaving the 

 quartz alone. I shall not enter into an inquiry respecting the 

 probability of such changes in mountain masses, of such an 

 enormous bulk as the dolomite of the Tyrol, to which Von 

 Buch ascribed a similar origin. The facts observed on a small 

 scale do not exclude the possibility of such changes, though 

 we are certainly less prepared to expect them, where powerful 

 and momentary revolutions are supposed to have taken place 

 at the same time, than where any period of time, even the most 

 protracted, may be granted for the successive replacement of 

 one particle of matter by another. 



Crystals of calcareous spar, previously coated with small in- 

 dividuals of quartz, often entirely disappear, and leave an 

 empty shell. We sometimes observe particles of the calcareous 

 spar with a corroded surface still contained within the cover-t 

 ing, but much diminished in size. A large pseudomorphosis 

 in the shape of a scalene six-sided pyramid, from the zinc mines 

 in Somersetshire, in Mr Allan's cabinet, from which the origi- 

 nal species of calcareous spar has entirely disappeared, is of a 

 particularly interesting nature. Beside the superficial coating, 

 the quartzy matter has introduced itself into the fissures of the 

 crystal, parallel to its planes of cleavage, and the interior of it 



