216 DESCRIPTIONS OF MINERALS. 



to leave a trace on a board. Forms mountain beds. Most 

 chalk was made chiefly out of the shells of Khizopods. 



Rock milk. White and earthy like chalk, but still softer, 

 and very fragile. It is deposited from waters containing 

 lime in solution. Rock 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. The so-called "Gibraltar rock" is 

 stalagmite from a cavern in the rock of Gibraltar. 



Limestone is a general name for all the massive varieties 

 occurring m extensive beds. 



Oolite, Pisolite. Oolite is a compact limestone, consist- 

 ing of small round concretionary grains, looking like the 

 spawn of a fish ; the name is derived from the Greek don, 

 an egg. Pisolite, a name derived from pisum, the Latin for 

 pea, differs from oolite 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- 

 minae a little waving, and containing some silica. 



Fontainebleau limestone. This name is applied to crys- 

 tals of the form shown \i\ 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 limestone. The limestones breaking with a 

 smooth 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 when polished are used 

 as marbles. A black color is common, and is usually due to 



