Al4 DESCRIPTIONS OF ROCKS. 
Rocks thus metamorphosed or rendered crystalline are 
distinguished as metamorphic rocks. 
C. By chemical deposition. Waters often hold calcareous 
material in solution. When carbonic acid (carbon dioxide) 
is present in any waters, those waters will take up calcium 
carbonate, and make calcium bicarbonate; and when the 
waters evaporate, the calcium carbonate is deposited. This 
is the process by which stalactites and stalagmites (p. 216) 
have been made, and so also calcareous tufa and travertine 
(p. 432). The Gardiner River region in the Yellowstone 
Park is noted for its deposits of travertine. 
In geyser regions there are siliceous deposits made by the 
hot waters, as stated on page 240; and these also are exem- 
plified in the Yellowstone Park. 3 
Beds of tripolite (p. 241) sometimes become consolidated 
and converted into chert by the waters that penetrate them 
—these waters containing a trace of alkali or enough to 
enable them to dissolve some of the tripoli silica, and then 
a deposition taking place causing consolidation. ‘The flint 
and chert of the rocks has probably had generally this ori- 
gin. 
3. CALCAREOUS Rocks or LIMESTONES. Compact lime- 
stones are commonly of fragmental origin. They have been 
made mainly out of worn or ground-up shells, corals, and like 
calcareous material of organic origin—the movements of the 
ocean having been, and still being, the grinding agency. 
They were consolidated through the ocean’s waters which 
penetrated the beds taking up a little calcareous material, 
and then depositing it again. It is, in one sense, metamor- 
phism. But when such compact limestones experience true 
metamorphism, at the same time with other strata, they be- 
come distinctly crystalline-granular, and often very coarsely 
so, making crystalline limestone or marble. 
lil. ON SOME CHARACTERISTICS OF ROCKS. 
1. CRYSTALLINE TEXTURE. Crystalline texture varies in 
coarseness from that in which crystalline grains are visible 
only under high magnifying power, and the rock is as apha- 
nitic (p. 60) as flint, to that in which they are very coarse. 
Not unfrequently one of the minerals appears in large crys- 
tals, distributed through the mass—the mass being made of 
