APRIL 14, 1911] 



SCIENCE 



561 



it may be said to be absent. Pyroxenie 

 minerals are rare and only in microscopic 

 crystals. Magnetite is 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 ilmenite and 

 pseudobrookite. Apatite, zircon, and al- 

 lanite complete the limited list of accessory 

 minerals. 



CLASSIFICATION AND COMPOSITION OF 

 THERMAL SPRINGS 



The number of springs scattered over 

 the park, from which flow varying 

 amounts of thermal waters, probably ex- 

 ceed twenty-five hundred. If to these be 

 added the fumaroles, solfataras, and nar- 

 row 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 be- 

 coming extinct. Furthermore, it would be 

 no easy task to decide 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 alka- 

 line waters rich in dissolved silica, (3) 

 calcic siliceous waters having both proper- 

 ties of calcic carbonate and siliceous alka- 

 line 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. Whitfield in the laboratory of the 



United States Geological Survey and the 

 results published in a separate bulletin.* 



Among these waters are several from 

 the Mammoth Hot Springs characterized 

 by the large amount of calcic carbonate in 

 solution, associated with free carbon diox- 

 ide 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 surface and 

 the diffusion of free carbon dioxide, pre- 

 cipitation followed, as shown in the de- 

 posits which have made the Mammoth Hot 

 Springs so famous. From the present 

 point of view we are not so much con- 

 cerned with depositions from these waters 

 as with the waters themselves and their 

 geological relations, since they unquestion- 

 ably have a common source with those of 

 the rhyolite plateau. 



At the Mammoth Hot Springs the upper 

 lava flows lie directly against inclined 

 Jurassic limestones. The circulating hot 

 waters having been diverted in their 

 course, traverse the limestone before is- 

 suing at the surface. Apparently the 

 waters derive a large part of their min- 

 eral constituents from the limestone. In 

 contrast to the hot waters of the plateaus 

 they carry but little silica in solution. 

 Far to the southward, where the rhyolite 

 tableland ends, the attenuated lava streams 

 also rest against uplifted limestone^. 

 Calcic carbonate springs, although of 

 modest dimension, issue through the rhyo- 

 lite, but have derived their mineral con- 

 tents mainly from the adjacent limestones. 

 Both the carbonated waters and the trav- 

 ertine deposits resemble those at Mam- 

 moth Hot Springs. Similar geological 

 relations may be observed near the eastern 



* F. A. Gooch and J. E. Whitfield, " Analyses of 

 Waters of Yellowstone National Park," Bull. U. 

 S. Geol. Survey No. 47, Washington. 



