STATE GEOLOGIST. 85 
respects and must be examined as to their composition and extent of the 
deposit. The ideal source of sand would be the material known as mold- 
ing sand, which is very fine grained, though carrying considerable iron and 
alumina. 
LIME, 
; A rough classification of lime materials might be made as follows: 
I. Crystalline calcium carbonate, crystalline character well developed. 
2. Dense, glistening. 
3. Dense, dull, non-crystalline, often impure. 
4. Amorphous, porous, earthy, often impure. 
The gradations from the chemically pure calcium carbonate which 
was formed under favorable natural conditions to the most impure calcar- 
eous materials, like the calcareous clays, are not abrupt. Sedimentation of 
clayey materials was always coincident with the precipitation of the lime 
to a greater or less degree, and hence we can look for pure carbonate only 
in exceptional cases. IN good deal of the pure lime is obtained by resolu- 
tion and precipitation, or by taking up of lime by animal life, resulting in 
the building up of banks of animal remains (coral banks). 
Crystalline Calctum Carbonate.—Calcium carbonate in its state of 
greatest purity is represented by calcite, whose specific gravity is 2.72 to 
2.90. Another form of crystalline calcium carbonate, the aragonite, crys- 
tallizing in the rhombic system, has a specific gravity of 2.9 to 3. Whileon 
burning calcite calcium oxide is obtained which slakes in water with the 
evolution of heat, aragonite when burnt increases apparently in volume 
and breaks up into an asbestos-like matted mass of needle-like crystals. 
When powdered and moistened with water the substance does not heat, 
but breaks down slowly to a powder. It is harder than calcite, but more 
soluble in water. 
The principal forms of crystalline calcium carbonate are the coral 
rocks, the white or grayish coarse-grained limestones, showing the glitter- 
ing calcite grains very distinctly, the calcareous masses deposited from 
solution in limestone caves (stalagmites, stalactites), travertine, etc. 
When metamorphosed by the action of heat and pressure, limestones may 
be changed by recrystallization to marbles, which are characterized by an 
extremely uniform grain, giving them a granular structure. 
The Dense, Glistening Forms.—The dense, glistening limestone rocks 
are as crystalline as the preceding materials, but the crystallization is not 
as well defined and the individual crystals are, as a rule, too small to be 
identified. These rocks form the most important source of lime for the 
lime industry,,and hence are of much greater economic value than the 
crystalline rocks of the first section. These limestone occur in massive 
beds and are extremely dense, producing the greatest amount of caustic 
