494 REPORT OF NATIONAL MUSEUM, 1900, 



Goov^ egg; pisolite differs from oolite in the larger size of its particles. 

 Stalactites are pendent masses of limestone formed in caverns by the 

 percolation of water, holding lime in solution, which on evaporation 

 leaves the carbonate of lime. Stalagmites are of the same material, 

 covering the floors of the cavern. Stalactites and stalagmites vary in 

 diaphaneity from nearly transparent to opaque, and are frequently 

 made up of layers of different colors, giving rise when polished to 

 agate-like bandings. Stalagmites when fine grained and pure white 

 are sometimes called alahasteQ'. Oriental onyx^ Gihraltar stotie, onyx 

 rnarhle, and Mexican onyx have beautif ullj^ banded, mottled, or clouded 

 structures, often showing wide ranges of color. 



Calcite and aragonite consist essentially of carbonate of lime (lime 

 56, carbonic acid 44). They occur crystallized and massive, and vary 

 in color from white through various shades of gra}^, red, green, ^^ellow, 

 brown, blue, and black. Calcite has a hardness of 3 and a specific 

 gravity of 2.72 when pure. Aragonite has a hardness of 3.5 and a 

 specific gravit}' of 2.93. Their use as gems is limited to fibrous kinds, 

 called satin spar, and those specimens showing rich colors. 



CANCRINITE. 



Cancrinite is occasionally fine enough to be used as a gem. It crys- 

 tallizes in six and twelve sided hexagonal prisms, also occurring 

 massive. Luster, weak vitreous, inclining to greasy. Transparent to 

 subtransparent. Color, yellow of sfeveral shades, also white, gray, 

 greenish, bluish, or reddish. Hardness 5.5 to 6; specific gravit}'^ 2.44. 



Cancrinite is found at Miask in the Urals; also in the Turkinsk 

 Mountains in a coarse granite with zircon, calcite, and magnetite; and 

 in crystals and massive at Litchfield, Maine, with sodalite and zircon. 

 This localit}^ affords bright orange yellow and pale yellow specimens; 

 occasionally the yellow cancrinite is found penetrating the blue sodal- 

 ite, forming, when polished, a very attractive stone. 



One hundred parts of cancrinite contain: Silica, 38.7; carbon diox- 

 ide, 6.3; alumina, 29.3; lime, 4; soda, 17.S; water, 3.9. 



CASSITERITE. 



WOOD TIN — TOAD'S-EYE TIN. 



Cassiterite, or tin stone, is used to a very limited extent for orna- 

 mental purposes. Its hardness is between 6 and 7; specific gravity 

 6.69; luster adamantine. Color, brown or black, occasionalh' gray, 

 white, or j'^ellow. The mineral occurs in tetragonal crystals and mass- 

 ive; often in reniform shapes having a fibrous structure. The variet}' 

 known as wood tin occurs in reniform or botryoidal shapes made up 

 of concentric layers or ])andings having a fibrous structure and resemb- 

 ling dark wood. Toad's-eye tin is the same on a smaller scale. Cas- 

 siterite contain}? in lOO parts: Tin, 78.28; oxygen, 21.62. 



