160 Naming Rocks. 
Tripoli-earth—A powdery rock, formed of minute 
frustules of diatom plants. 
Limestone—A grayish, yellowish or brownish rock 
of various degrees of purity. It is, when pure, formed 
of calcium carbonate which has been precipitated from 
water holding lime in solution. H.=3. G.=2.25—2.75. 
Stalactites—These are pendent, and stalagmites are 
upright, limestone. They may be of any size, from a 
mere thread up to a solid pillar many feet in length 
and diameter. They are the result of dripping lime 
water. 
Marble—A granular or crystalline limestone, due 
to metamorphism. Of various colors, from white to 
gray, with reddish and other tints. Impurities are 
mica, tremolite, pyroxene, scapolite, pyrite, serpentine, 
chlorite, spinel, graphite, etc. Varieties—Calcite, Dolo- 
mite and Calcite-Dolomite Marble. 
Calcareous-tufa (travertine )—A lime-carbonate de- 
posit, formed by springs issuing from limestone and it 
is a sediment precipitated from their waters. 
#H ydraulic liomestone—Contains a small portion of 
clay and has the property of hardening under water after 
being calcined or burnt. 
Dolomyte (magnesium limestone)—A dirty grayish 
or yellowish rock. When pure, it consists of 54 per cent 
magnesium-carbonate and 46 per cent. of calcium car- 
bonate. It is compact, but often assumes globular or 
other concretionary forms. «It is harder than limestone 
and does not effervesce so freely in acids. When meta- 
morphosed it makes an impure marble, frequently show- 
_ing veins of iron oxide running through the ground 
mass. 
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