54 STRUCTURE OF MINERALS. 



Stalactitic ; having the form of a cylinder, or cone, hang- 

 ing from the roofs of cavities or caves. The term stalactite 

 is usually restricted to the cylinders of carbonate of lime 

 hanging from the roofs of caverns : but other minerals are 

 said to have a stalactitic form when resembling these in theii 

 general shape and origin. Chalcedony and brown iron ore 

 are often stalactitic. 



Reticulated ; net-like. 



Drusy ; a surface is said to be drusy when covered with 

 ninute crystals. 



Amorphous ; having no regular structure or form, either 

 crystalline or imitative. The word is from the Greek, and 

 means without shape. 



PSEUDOMORPTIOUS CRYSTALS. 



A pseudomorphous* crystal is one that has a form which is 

 foreign to the species to which the substance belongs. 



Crystals sometimes undergo a change of composition from 

 aqueous or some other agency, without losing their form ; 

 for example, octahedrons of spinel change to steatite, still 

 retaining the octahedral form. Cubes of pyrites are changed 

 to red or brown iron ore. 



Again : crystals are sometimes removed entirely, and at the 

 same time and with equal progress, another mineral is sub- 

 stituted ; for example, when cubes of fluor spar are trans- 

 formed to quartz. The petrifaction of wood is of the same 

 kind. 



Again : cavities left empty by a decomposed crvstal, are 

 refilled by another species by infiltration, and the new 

 mineral takes on the external form of the original mineral, 

 as a fused metal the form of the mould into which it is cast. 



Again : crystals are sometimes incrusted over by other 

 minerals, as cubes of fluor by quartz ; and when the fluor is 

 afterwards dissolved away, as sometimes happens, hollow 

 cubes of quartz are left. 



The first kind of pseudomorphs, are pseudomorphs by al- 

 teration ; the second, pseudomorphs by replacement ; the 



What is a pseudomorphous crystal ? What is the first, the second, 

 the third and the fourth mode of pseudomorphism? Wnat are they 

 wiled? 



* From the Greek pseudes, false, and morphe, form. 



