HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 



529 



is again deposited as carbonate. In this way are formed numerous and 

 at times extensive deposits, to which are given varietal names depend- 

 ent upon their structure and the special conditions under which they 

 originated. Calc sinter or tufa is 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 

 snow-white color, as seen in specimens 12882, 72871, and 72876, from the 

 Mammoth Hot Springs of the Yellowstone National Park. Somewhat 

 similar deposits are shown from springs in Virginia (35759) ; California 

 (29637) ; Mexico (37787), and New Zealand (70335). Others, like those 

 from Niagara Falls, New York, and Soda Springs, Idaho (36107, 39136), 

 were formed by the deposition of the lime on leaves and twigs, form- 

 ing beautifully perfect casts of these objects. 



Tufa deposits of peculiar imitative shapes have been described by 

 Mr. I. 0. Russell of the U. S. Geological Survey, as formed by the evap- 

 oration of the waters of Pyramid Lake, Nevada (35260). See, also, col- 

 lection from Lake Lahantou, Nevada, in fbjor upright case). Oolitic 

 and pisolitic limestones are so called on account of their rounded fish- 

 egg like structure, the word oolite being from the Greek word won, an 

 egg. (See PI. cxxi.) These are in part chemical and in part mechanical 

 deposits. The water in the 

 lakes and seas in which they 

 were formed became so satu- 

 rated that the lime was depos- 

 ited in concentriccoatings about 

 the grains of calcareous sand on 

 th e bottom , and finally the li ttle 

 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. 91. Samples are shown 

 such as are now forming in Pyr- 

 amid Lake, Nevada (No. 35378) ; 

 Cache Valley, Utah (35305 and 

 and 35306) ; Great Salt Lake, 

 Utah (35379), and Key West, 

 Florida (18708), and from other geological formations of America and 

 Europe (Nos. 36115 and 36960). Only those which are largely chem- 

 ical are here shown ; others are to be found in the group of stratified 

 rocks. 



Travertine is a compact and usually crystalline deposit formed, like 

 the tufas, by waters of springs and streams. The travertines are often 



Fig. 91. 



MlCKO-STKUCTUItE OF OOLITIC LlMESlONK. 



(Princeton, Kentucky.) 



* See Mr. Weed'8 paper on the Formation of Travertine and Siliceous Sinter, Annual 

 Report United States Geological Survey for 1337-88. 



H. Mis. 129, pt. 2— -34 



