216 DESCRIPTIONS OF MINERALS. 
to leave a trace on a board. Forms mountain beds. Most 
chalk was made chiefly out of the shells of Rhizopods. _ 
Rock milk. White and earthy like chalk, but still softer, 
and very fragile. It is deposited from waters containing 
lime in solution. Lock meal is a powdery variety. 
Calcareous tufa. Formed by deposition from waters like 
rock milk, but more cellular or porous and not so soft. 
Stalactite, Stalagmite. ‘The name stalactite is explained 
on page 60. The deposits of the same origin that cover 
the floors of caverns are called stalagmite. They generally 
consist of differently colored layers, and appear banded or 
striped when broken. ‘he so-called ‘‘ Gibraltar rock” 1 
stalagmite from a cavern in the rock of Gibraltar. 
Limestone is a general name for all the massive varieties 
occurring in extensive beds. 
Oolite, Pisolite. Qdlite is a compact limestone, consist- 
ing of small round coneretionary grains, looking hke the 
spawn of a fish; the. name is derived from the Greek 501, 
an egg. Pisolite, a name derived from piswm, the Latin for 
pea, differs from odlite in being coarser; the spherules 
often have a concentric structure, and thus show their con- 
cretionary origin. : 
Argentine. A white shining limestone consisting of la- 
mine a little waving, and containing some silica. 
Fontainebleau limestone. This name is applied to crys- 
tals of the form shown in figure 3, containing a large pro- 
portion of sand, and occurring in groups. They were for- 
merly obtained at Fontainebleau, France, but the locality 
is exhausted. 
Granular limestone. A limestone consisting of crystal- 
line grains, and hence often called crystalline limestone. 
The coarser varieties when polished constitute the common 
white and clouded marbles, and are the material of which 
“marble” buildings are made. The finer are used for 
statuary, and are called statuary marble. The best is as 
clear and fine-grained as loaf sugar, which it much re- 
sembles. 
Compact lmestone. The limestones breaking with a 
smcoth surface, without a distinctly granular texture, and 
dull in lustre unless polished. ‘The rock is very variously 
colored. ‘The colors are sometimes arranged in blotches, or — 
veins. Kinds that are handsome bite ‘polished are used 
as marbles. A black color is common, and is usnally due to 
