114 A. HAGUE THERMAL WATERS IN YELLOWSTONE PARK 



somewhat more widely distributed, but only in minute grains, as might 

 be supposed from the low percentage of iron oxides. The presence of 

 titanium in the magma reveals itself in the disseminated grains of ilme- 

 nite and pseudobrookite. Apatite, zircon, and allanite complete the 

 limited list of accessory minerals. 



Classification and Composition of thermal Springs 



The number of springs scattered over the park, from which flow vary- 

 ing amounts of thermal waters, probably exceed twenty-five hundred. If 

 to these be added the fumaroles, solfataras, and narrow rifts, from which 

 issue steam and gaseous emanations, mingled with more or less water, 

 the number would be greatly augmented. It is impossible to enumerate 

 them, as new ones are frequently reaching the surface, while others are 

 slowly becoming extinct. Furthermore, it would be no easy task to de- 

 cide whether single points of discharge should be counted or considered 

 as a group having a common source a short distance below ground. These 

 thermal waters reach the surface holding mineral matter in solution, de- 

 rived from the decomposition of rocks through which they pass in their 

 upward movement. They may be arranged under four heads : 



1. Waters carrying calcic carbonate in solution. 



2. Siliceous alkaline waters rich in dissolved silica. 



3. Calcic siliceous waters having both properties of calcic carbonate 

 and siliceous alkaline springs. 



4. Siliceous acid waters, usually holding free acid in solution. 

 Nearly thirty of these thermal waters have been analyzed by F. E. 



Gooch and J. E. AVhitfield in the laboratory of the United States Geo- 

 logical Survey and the results published in a separate bulletin.* 



Among these waters are several from the Mammoth Hot Springs char- 

 acterized by the large amount of calcic carbonate in solution, associated 

 with free carbon dioxide and sulphates of magnesium and the alkalies. 

 Underground conditions were doubtless favorable for holding in solution 

 large amounts of calcic carbonate. With the relief of pressure at the sur- 

 face and the diffusion of free carbon dioxide, precipitation followed, as 

 shown in the deposits which have made the Mammoth Hot Springs so 

 famous. From the present point of view we are not so much concerned 

 with depositions from these waters as with the waters themselves and 

 their geological relations, since they unquestionably have a common 

 source with those of the rhyolite plateau. 



At the Mammoth Hot Springs the upper lava flows lie directly against 



* P. a. Gooch and J. E. Whitfield : Analyses of waters of Yellowstone National Park. 

 Bulletin of the U. S. Geological Survey, No. 47, Washington. 



