414 DESCRIPTIONS OF EOCKS. 



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. 



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 Limestoxes. 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. 



HI. OX 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. 

 Xot unfrequently one of the minerals appears in large crys- 

 tals, distributed through the mass — the mass being made of 



