86 Observations on Pseudomorphism. 



The change of datholiie to chalcedony, (Haytorite,) we are 

 inclined to place in the same category with the above, although 

 considered by Dr. Blum, an instance of alteration rather than 

 replacement. 



PSEUDOMORPHS BY ALTERATION. 



The pseudomorphs by alteration, although seemingly rendered 

 more intelligible by means of the chemical formulas than those 

 we have just considered, are yet in many cases more difRcult of 

 satisfactory explanation. 



Before proceeding with our remarks upon them, we would di- 

 rect attention to the small number of minerals constituting these 

 pseudomorphs, as shown in the catalogue given on pp. 67, 68. 

 Of earthy species, there are only the following ; calc spar, gyp- 

 sum, heavy spar, Dolomite, mica, kyanite, hornblende, kaolin, 

 cimolite, Prehnite, the magnesian minerals, steatite, talc, serpen- 

 tine, and chlorite, and rarely quartz with some of its varieties, 

 and opal. 



For the elucidation of these pseudomorphs, we have above 

 distributed them into, 



(1.) Those formed at the ordinary temperature ; and 



(2.) Those requiring an elevated temperature. 

 I. Pseudomorphs formed at the ordinary temperature. 



Pseudomorphs of the first of these divisions, result from the 

 agency of the elements of the atmosphere in connection with 

 moisture ; but the process is usually more or less dependent on 

 foreign substances in solution in water, such as its common im- 

 purities, salts of lime, carbonic acid, &c., and in many instances 

 the results of mineral decomposition in the vicinity, aid in produ- 

 cing the change. They may be farther subdivided as follows ; 



1. Those resulting simply from atmospheric agents and moisture ; 



2. Those produced by the agency of gases, salts or acids con- 

 tained in the water or atmosphere. 



The following are a few examples in illustration of the first of 

 these subdivisions. 



Anhydrite to Gypsum,. — A simple addition of atmospheric mois- 

 ture. 



Specular Iron to Brown Iron Ore. — The same. 



Red Copper Ore to Malachite. — Combination with the carbonic 

 acid of the atmosphere and moisture. 



