83 Observations on Pseudomorphism. 



3. By Replacement ; in which one mineral gradually replaces 

 another, assuming at the same time its form, without any inter- 

 change of elements; for example, the siliceous cubes, psendo- 

 morphs of fluor spar, in which silica replaces the original fluor, 

 and has taken on its cubic form. Here the process is in a certain 

 sense chemical, and is wholly different from the simple act of 

 deposition, by which the first two kinds of pseudomorphs above 

 mentioned, were formed. 



These three so different classes of pseudomorphs are included 

 together by Dr. Blum, and constitute his second section, headed 

 " Verdrangung-Pseudomorphosen," or pseudomorphs by expul- 

 sion. 



4. By Alteration ; in which some of the elements of the 

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

 Thus anhydrite becomes gypsum by the addition of water ; feld- 

 spar becomes kaolin by the addition of water, and removal 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 reference to 

 the formation of pseudomorphs. The change of hornblende to 

 steatite, placed in \\h first subdivision, is undoubtedly due to the 

 same cause that produced the change of augite, feldspar, mica, 

 &c., to steatite, given in his i/i2Vc? subdivision ; and the first men- 

 tioned, should be placed, in a natural arrangement, near the latter. 

 The following distribution is proposed : but much further inves- 

 tigation is required before it can be fully carried out. 



a. Pseudomorphs that are formed at the ordinary temperature, 

 and result from atmospheric agents. 



h. Pseudomorphs that proceed from the same and other agents, 

 but only under the action of heat. 



5. By Allomorphism ; in which a substance nnder one of its 

 dimorphous forms, changes to the other without altering the ex- 

 ternal form. 



The processes by which pseudomorphs of the first two kinds 

 {hy Infiltration and by Incrustation) have resulted, are too simple 



