J. D. Dana on Pseudomorphism. 253 



original mineral are removed, or exchanged, or others added. 

 Thus anhydrite becomes gypsum by the addition of water ; 

 felspar becomes kaolin by the addition of water, and re- 

 moval of potash and some silica. This class includes all the 

 pseudomorphs in Dr Blum's first section, his "Umwandlungs- 

 Pseudomorphosen," or pseudomorphs by exchange. The 

 change producing them is wholly chemical, being the result 

 of chemical affinities. 



Dr Blum's subdivision of this class, although convenient 

 for description, is objectionable when considered with refer- 

 ence to the formation of pseudomorphs. The change of horn- 

 blende to steatite, placed in his first subdivision, is undoubt- 

 edly due to the same cause that produced the change of 

 augite, felspar, mica, &c., to steatite, given in his third sub- 

 division ; and the first-mentioned should be placed, in a 

 natural arrangement, near the latter. The following distri- 

 bution is proposed, but much further investigation is required 

 before it can be fully carried out : — 



a. Pseudomorphs that are formed at the ordinary tempera- 

 ture, and result from atmospheric agents. 



b. Pseudomorphs that proceed from the same and other 

 agents, but only under the action of heat. 



5. Bi/ Allomorphosm ; in which a substance, under one of 

 its dimorphous forms, changes to the other without altering 

 the external form. 



The processes by which pseudomorphs of the first two 

 kinds (by infiltration and by incrustation) have resulted, are 

 too simple to require further remark. We shall allude, on 

 a following page, to the causes that may have been instru- 

 mental in removing the crystals. 



PSEUDOMORPHS BY REPLACEMENT. 



The pseudomorphs by replacement, are considered the most 

 singular of altered crystals. In some way, the substance of 

 a crystal is entirely removed, and, at the same time, is re- 

 placed by some other material, without changing an angle or 

 plane. Yet these altered crystals appear to be little more 

 singular in their mode of formation than the common petri- 

 faction of wood. The particles of wood, as they pass off, are 



