\ 



^* 





240 



MINERALOGY. 



[Sect. VIII, 



in our present state of knowledge so to divide them. The 

 fiFst class shows the mere filling of a mould, left by the 

 disappearance of one mineral, with the matter of another 

 of dissimilar chemical composition, and which, if it could 

 have crystallized out freely in a cavity, or amid yieldin*^ 



a change of conditions took 



particles of matter, w^ould also have had a dissimilar ex- 

 ternal form. The study of the minerals forming the 

 substances which have filled up the cracks or dislocations 

 in rocks, commonly known as mineral veins, or fault>, as 

 they may or may not contain ores of the useful metals, 

 abundantly shows us that, after the formation in them of 

 some crystallized minerals, and the envelopment of these 

 by another mineral substance, 

 place in the crack or fissure, in such cases only partially 

 filled wdth solid matter, so that the original crystals were 

 removed. It is again obvious that while the cavities left 

 by these crystals continued empty, or formed little hol- 

 low^s in which minerals of various kinds have merely 

 covered the sides, partly filling these hollows, at others 

 some given mineral has completely occupied the cavities, 

 so that while internally keeping the structure peculiar to 

 it, the exterior form corresponds exactly with that of the 

 original and removed crystal. Further observation shows 

 us that while in the greater number of instances the mine- 

 ral substance forming the mould has remained firm, fre- 



quently covered over by other mineral substances, depo- 

 sited from variable solutions in the crack or fissure until 

 this bo finally and completely filled with solid matter ; 

 sometimes the mould has been dissolved and removed, the 

 pseudomorphous or externally false-shaped crystals ap- 

 pearing uncovered and by them.selves. At least they so 



t 



Se«^ 



fill. 



,01^ 



re 



those 



5 



cba 



-A'l',. 



ar 



J 



■^n. 



'0^ 



were, <^' 

 clialcedon 



h 



-1^1 other 

 tl . fill 



15; 



vt '■\r. 



i 



S 



ucn a^ . 



•1 



minerak : 

 those cl(=^n 



and tbr 



'^\^ 



parati^'^ 



'm of ^ 



tliat 



W. 



we^ 



- 1] 



^ par- 



1 ^^-jji ^ 



