252 J. D. Dana on Pseudomorphism. 



rests upon this subject, yet we may ventm^e to offer some 

 considerations in its elucidation, such as appear to result 

 from the known causes acting in nature, and the facts that 

 have passed in review. Could we believe, with some late 

 chemists, in the transmutation of the elements, we should 

 consider this a fruitless subject of investigation, since, in the 

 midst of the many possibilities which an hypothesis creates, 

 we should hardly know which to choose. But the received 

 principles of chemical science are believed sufficient for these 

 explanations, without the aid of such speculations. 



Viewing pseudomorphs with reference to their origin, we 

 may make the following distribution of them ; those pro- 

 duced, 



1. By Infiltration ; in Avhicli a cavity previously occupied 

 by a crystal, is refilled by some other mineral. A cubic 

 crystal of salt in clay is removed by percolating watei', and 

 the cavity afterwards filled with gypsum. In this there is 

 nothing chemical ; it is simply a mechanical deposition into 

 a ready formed mould. 



2. By Incrustation ; in which a mineral incrustation covers 

 a crystal, which crystal is subsequently removed by some 

 process of solution. Like pseudomorphs by infiltration, this 

 process is mechanical — a simple deposition of foreign matter 

 around a crystal. The process by which the original crystal 

 was removed is a separate inquiry, and one of much interest. 



3. By Eeplacement ; in which one mineral gradually re- 

 places another, assuming, at the same time, its form, without 

 any interchange of elements ; for example, the siliceous cubes, 

 pseudomorphs of fluor-spar, in which silica replaces the ori- 

 ginal fluor, and has taken on its cubic form. Here the pro- 

 cess is, in a certain sense, chemical, and is wholly difi'erent 

 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 in- 

 cluded together by Dr Blum, and constitute his second sec- 

 tion, headed " Verdrangung-Pseudomorphosen," or pseudo- 

 morphs by expulsion. 



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



