CARBONATES 



105 



a loose friable deposit made by springs and streams either by 

 evaporation or through intervention of algous vegetation. Such 

 are often beautifully arborescent and of a white color, as seen 

 at the Mammoth Hot Springs of the Yellowstone National Park. 

 Somewhat similar deposits are formed by springs in Virginia, 

 California, Mexico, New Zealand. 



Tufa deposits of peculiar imitative shapes have been described 

 by Mr. I. C. Russell of the United States Geological Survey, 

 as formed by the evaporation of the waters of Pyramid Lake, 

 Nevada. Oolitic and piso- 

 litic limestones are so 

 called on account of their 

 rounded, fish-egg-like 

 structure, the word oolite 

 being from the Greek 

 work oiov, an egg. (See 

 PL 11.) These are in 

 part chemical and in part 

 mechanical deposits. The 

 water in the lakes and 

 seas in which they were 

 formed became so satur- 

 ated that the lime was 

 deposited in concentric 

 coatings about the grains 

 of calcareous sand on 

 the bottom, and finally the little granules thus formed became 

 cemented into firm rock by the further deposition of lime 

 in the interstices. This structure will be best understood by 

 reference to Fig. 7. Eoeks of this nature are now forming along 

 the beaches of Pyramid Lake. 



Such forms as these may or may not show a nucleus. It 

 seems safe to assume that such a nucleus, at first, in all cases 

 existed, though it may be in microscopic dimensions only. 



Travertine is a compact and usually crystalline deposit formed, 

 like the tufas, by waters of springs and streams. The traver- 

 tines are often beautifully veined and colored by metallic oxides 

 and form some of the finest marbles. Such are the so-called 

 ''onyx marbles" of Mexico and Arizona.^ 



=^Tlie Onyx Marbles, Ann. Eep. U. S. National Museum for 1893. Also 

 Stones for Building and Decoration, Wiley & Sons, ISTew York, 2d ed., p. 120. 



Pig. 7. — Microstructure of oolitic limestone. 



