274 SCIENCE PROGRESS. 



For these reasons the subject of mineral transformations 

 has been an exceedingly fruitful one for speculation, with 

 the inevitable result that what was originally a mere guess 

 has often become stereotyped into a positive assertion, and is 

 accepted without question or criticism. 



In one class of mineral substances alone are we on fairly 

 sure ground. 



Possessing a certain individuality, a crystallised mineral 

 may under some conditions, although partially changed, re- 

 tain sufficient characteristics for its identification ; in the re- 

 markable class of products known as " pseudomorphs," the 

 identification is often possible even when the original mineral 

 has been entirely converted into something different. 



A mineral which is stable under one set of conditions, is 

 frequently unstable under others ; when exposed to the 

 action of air, water, or various solvents it may be con- 

 verted into new substances, and if the action has been 

 sufficiently gradual, the resulting product may preserve 

 intact the crystalline form of the mineral from which it 

 originated. In such a case there can be absolutely no 

 doubt that it was produced by the alteration of that parti- 

 cular mineral, and although the mode of its production may 

 be doubtful, its origin is certain. 



Such perfect pseudomorphs, for example, as those found 

 at Wunsiedel in Bavaria, which, while consisting of pure 

 Steatite, present the characteristic and unmistakable form of 

 Quartz, can obviously have resulted only from the alteration 

 or destruction of Quartz crystals. Hence the special and 

 unique interest of such "pseudomorphs," which are, in fact, 

 the only safe guide to a study of mineral transformations. 



There are, of course, two possibilities to be kept in view; 

 such pseudomorphs may have been produced by the altera- 

 tion of the parent mineral, or they may be mere casts of the 

 same. In the latter case we can only conclude that the 

 original crystal has been removed by the action of some 

 solvent, and that its place has been filled by the deposition 

 of new material. Such replacements need not necessarily 

 throw any light upon the constitution either of the initial 

 substance or of the resulting product. Many, possibly the 



