ROCKS 65 
Tufa or Calc-sinter is formed by deposition from 
calcareous springs. It is a porous and frequently very 
friable compound of calcium carbonate. The colour varies ; 
creamy-white and yellow tints are common, but red and 
brown are not infrequent, while some kinds are greenish or 
bluish. The -rock is often mottled or marked with concentric 
bands of different colours. 7vavertine is the name given to 
hard and compact varieties used for building-stoncs. They 
have frequently a crystalline or subcrystalline structure. 
Some tufas and travertines consist largely of small spherules 
of calcium carbonate, composed of concentric layers which 
have been deposited successively around some nucleus, such 
as a particle of sand or a minute fragment of calcareous matter. 
When the spherules are small, resembling fish-roe, the rock 
is termed Oolzte ; when they are of the size of peas the rock 
is known as P2soliée. 
Dolomite or Magnesian Limestone is a crystalline 
granular or earthy aggregate of the mineral dolomite or 
-bitter-spar (double calcium and magnesium carbonate). It 
effervesces only slightly with cold dilute hydrochloric acid, 
but is readily attacked when the acid is heated. Ferrous 
carbonate and various impurities are more or less commonly 
present. When such is the case the rock is usually yellowish ; 
when impurities are only sparingly present it is grey or white. 
It frequently assumes a concretionary structure, showing 
botryoidal and irregular shaped masses, or appearing as if 
built up of spherical bodies that may vary in size from small 
marbles up to large cannon-balls. Lines of bedding pass 
through these curious concretions. Many irregular shaped 
cavities appear in dolomite, and these are often lined with 
crystals of bitter-spar. Typical dolomite is easily distin- 
suished from ordinary limestone by its superior hardness 
(3-5 to 4:5), its greater specific gravity (2-8 to 2-9), and its 
much less ready solubility in cold acid. 
Some dolomites, especially those which are associated with beds and 
layers of rock-salt and gypsum, may be of the nature of chemical precipi- 
tates on the floors of salt-lakes, lagoons, and other bodies of highly saline 
water. Many, however, would appear to have been originally common 
limestones which, either at the time of their formation or subsequently, 
have by various chemical processes been converted into magnesian 
E 
