

5 
Parr. §ii] | ROCK-FORMING MINERALS. 83 
the increased solvent power which it thereby acquires, and from the 
abundance of calcium in various forms among minerals and rocks, it is 
natural that calcite should occur abundantly as a pseudomorph 
replacing other minerals. Thus it has been observed taking the place 
of a number of silicates, as orthoclase, oligoclase, garnet, augite, and 
several zeolites; of the sulphates, anhydrite, gypsum, barytes, and 
celestine; of the carbonates, aragonite, dolomite, cerussite; of the 
fluoride, fluor-spar; and of the sulphide, galena. Moreover, in many 
massive crystalline rocks (diorite, dolerite, &c.), which have been long 
_ exposed to atmospheric influence, this mineral may be recognised by 
the brisk effervescence produced by a drop of acid, and in microscopic 
sections appears filling the crevices, or sending minute veins among 
the decayed mineral constituents. Calcite is likewise the great 
petrifying medium; the vast majority of the animal remains found 
in the rocky crust of the globe have been replaced by calcite, some- 
_ times with a complete preservation of internal organic structure, 
sometimes with a total substitution of crystalline material for that 
structure, the mere outer form of the organism alone surviving. 
Aragonite. Orthorhombie, also globular, columnar, fibrous, sta- 
lactitic, and encrusting. H. 3:5—4. Gr. 2°9—3. Composition same 
as calcite. The cause of the crystallization of calcium carbonate in 
the form of aragonite rather than calcite is still uncertain. Aragonite 
differs from calcite in being harder and heavier. It is much less 
abundant than the latter mineral, which is the more stable form of 
this carbonate. It occurs with beds of gypsum, also in mineral veins, 
in strings running through basalt and other igneous rocks, and in the 
shells of many mollusca. It is thus}always a deposit from water, 
sometimes from mineral springs, sometimes as a result of the 
- internal alteration of rocks, and sometimes through the action of 
living organisms. Being more easily soluble than calcite, it has no 
doubt. in many cases disappeared from limestones originally formed 
mainly of aragonite shells, and has been replaced by the more durable 
calcite, with a consequent destruction of the traces of organic origin. 
Hence what are now thoroughly crystalline limestones may have 
been formed by a slow alteration of such shelly deposits. 
Dolomite (Bitter-spar). Rhombohedral and isomorphous with 
calcite, the crystals usually visible only in open spaces of rocks; 
but most frequently granular and amorphous. H.35—45. Gr. 2°85 
_ —2:95. Composition—calcium carbonate 54°35, magnesium car- 
bonate 45-65, but these proportions are not constant, and the mineral 
is liable also to contain some ferrous or manganous carbonate. 
~ 
_ 
Only slowly acted on with little or no effervescence by cold acids, 
but when powdered soluble in warm acid. Occurs (1) as an original 
formation in massive beds (magnesian limestone) belonging to many 
different geological formations; (2) as a product of alteration, 
especially of ordinary limestone % of aragonite (p. 304). 
Siderite (Brown Ironstone Spathic Iron, Chalybite). Rhom- 
bohedral, with curved cleavage faces, also common in finely fibrous 
Gq 2 
