J. D. Dana on Pseudomorphism. 257 



though considered by Dr Blum an instance of alteration 

 rather than replacement. 



PSEUDOMORPHS BY ALTERATION. 



The pseudomorphs by alteration, although seemingly ren- 

 dered moi"e intelligible by means of the chemical formulas 

 than those we have just considered, are yet, in many cases, 

 more difficult of satisfactory explanation. 



Before proceeding with our remarks upon them, we would 

 direct attention to the small number of minerals constituting 

 these pseudomorphs, as shewn in the catalogue given in 

 pp. 241, 242. Of earthy species there are only the following : 

 — Calc-spar, gypsum, heavy-spar, dolomite, mica, kyanite, 

 hornblende, kaolin, cimolite, prehnite, the magnesian mi- 

 nerals, steatite, talc, serpentine, and chlorite, and rarely 

 quartz, with some of its varieties, and opal. 



For the elucidation of these pseudomorphs, we have above 

 distributed them into — 



I. Those formed at the ordinary temperature ; and, 



II. Those requiring on elevated temperature. 



1. 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 com- 

 mon impurities, salts of lime, carbonic acid, &c., and in many 

 instances the results of mineral decomposition in the vicinity, 

 aid in producing the change. They may be farther sub- 

 divided as follows : — 



1. Those resulting from atmospheric agents and moisture. 



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

 contained in the water or atmosphere. 



Tlie following are a few examples in illustration of the 

 first of these subdivisions. 



Anhydrite to Gypsum. — A simple addition of atmospheric 

 moisture. 



Specular Iron to Brorvn Iron Ore. — The same. 



VOL. XXXIX. NO. LXXVJII. — OCTOBEK 1845. R 



