62 CETSTALLOGEAPHT. 



calcite was dissolved away by the corrosive solvent, and took 

 its place as the calcite particles were removed. 



In all the above cases the pseudomorphs were made by simple 

 removal and cotemporaneous substitution. 



Again, crystals of the form of chrysolite, a magnesium sili 

 cate, occur, altered to serpentine, a hydrous magnesium silicate. 

 II Bre the pseudomorph was made by a process of alteration, 

 part of the ingredients remaining, and only water added. 



Again, crystals of siderite (spathic iron or iron carbonate) 

 occur changed to limonite, a hydrous iron oxide. Here there 

 was an oxidation of the iron of the carbonate, and the addition 

 of water. This is another example of pseudomorphs by altera- 

 tion. Similarly orthoclase changes to kaolin, and kaolin has 

 the form at times of orthoclase crystals. 



Again, crystals of the form of those of common salt occur 

 consisting of clay or of calcite, which were made by deposition 

 in a cavity left by the dissolving away of an imbedded crystal 

 of salt. These are pseudomorphs by deposition. 



Again, crystals of aragonite, i^rismatic calcium carbonate, 

 occur consisting of calcite or rhombohedral calcium carbon- 

 ate ; and here there is a change in crystallization without any 

 change of chemical composition. 



7. Fracture. — Kinds of fracture in these crystalline aggre- 

 gates depend on the size and form of the particles, their cohe- 

 sion, and to some extent their having cleavage or not. 



Among granular varieties, the influence of cleavage is in all 

 cases very small, and in the finest almost or quite nothing. The 

 term hackly is used for the surface of fracture of a metal, when 

 the grains are coarse, hard, and' cleavable, so as to be sharp 

 and jagged to the touch ; even, for any surface of fracture when 

 it is nearly or quite flat, or not at all conchoidal ; conchoidal, 

 when the mineral, owing to its extremely fine or cryptocrystal- 

 iine texture and hardness, breaks with shallow concavities and 

 convexities over the surface, as in the case of flint. The word 

 conchoidal is from the Latin concha, a shell. These kinds of 

 fracture are not of much importance in mineralogy, since they 

 distinguish varieties of minerals only, and not species. 



