SANDSTONES LIMESTONES DOL OMITE. 101 



Pumicestone. 



A spongy, porous, volcanic rock, usually, though not 

 always, greyish white or of some light colour. Floats on 

 water, although the powder has a specific gravity above 2. 

 Is very brittle. Before B.F. melts to a white enamel. In 

 composition, nearly the same as obsidian. Hardly acted on 

 by acids. 



Sandstones. 



These rocks may always be recognised by their appear- 

 ance, being made up of particles of sand cemented together. 

 The grains (chiefly silica) are very hard. Do not effervesce 

 in acid. 



Limestones. 



Eocks chiefly composed of carbonate of lime, and conse- 

 quently, like other carbonates, effervesce when a little hydro- 

 chloric acid is dropped on. Though infusible before the 

 B.F., limestone glows with a very bright light. Varieties : 



Chalk Soft, earthy, whitish and without lustre. 



Granular or compact limestone. 



Oolite, which consists of spherical grains, and in appear- 

 ance like the roe of a fish. Marly limestone, marble, 

 cole spar, &c. 



Dolomite. 



A colourless, white, sometimes yellow, green, or pale red 

 mineral, of a pearly, resinous, or vitreous-like appearance, is 

 composed of carbonates of lime and magnesia. Is infusible 

 before the B.F., but glows with a bright light. Though a 

 carbonate, does not effervesce much in acid. 



Clays. 



Contain usually about 40 to 50 per cent, silica, 30 alu- 

 mina ; water as well, sometimes, as iron, lime, potash, &c. 

 When mixed with water clays may be kneaded by hand into 

 various shapes. Usually, when in a dry state, very ab- 

 sorbent of water. Become hard when dried. Adhere to the 

 tongue. Some clays give out a disagreeable earthy smell 

 when breathed upon. Are generally infusible in a furnace. 



