62 CRYSTALLOGRAPHY. 
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. 
Bere 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 ef 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, prismatic 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. Hracture.—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 sur face of fracture when 
it is nearly or quite flat, or not at ail conchoidal ; conchoidal, 
when the mineral, owing to its extremely fine or cryptocrystal- 
line 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. 
