DURATION OF THERMAL ACTIVITY 109 



open valley of the Yellowstone. On the recession of the glacier frag- 

 ments of crystalline rocks, undoubtedly brought down from the Gallatin 

 country, were left strewn over the travertine of Terrace Mountain. 



It is a fair assumption that if these thermal waters were issuing 

 through rhyolite at one locality in pre-Glacial time, similar hot waters 

 and gaseous emanations should have reached the surface at other points 

 on the plateau. If any such remnants of sinters still remain it seems 

 impossible, from our present knowledge, to discriminate between them 

 and those of post-Glacial time. Erosion has carried away not only every 

 trace of these earlier deposits, but has removed nearly all evidences of 

 pre-Glacial rock decomposition. Modifications in topographical relief 

 fail to indicate two distinct periods, owing probably to the relatively 

 slight deposition of sinter before the ice. 



Following the withdrawal of a broad ice-sheet, ascending heated 

 waters, acting with renewed energy on the walls of innumtrabJe fissures 

 and rifts, bleached and kaolinized massive blocks of rock. This decom- 

 position of plateau lavas proceeded on a grand scale and left an indelible 

 impression on the rh5^olite area. In regard to the age of the hot springs, 

 it is reasonable to conclude that thermal waters were as active at the 

 close of the rhyolite extrusions as at any subsequent period. The an- 

 tiquity of many localities of decomposed rhyolite is clearly evident, as 

 shown by post-Glacial sculpturing. In certain areas where hydrothermal 

 energy was formerly a long-continued process, evidence of the presence 

 of such sources of heat have long since ceased. No one who has studied 

 the gradual development of these decompositions and metasomatic 

 changes under the influence of acid solfataras, or the deposition of sinter 

 now taking place from alkaline siliceous waters, can doubt the lapse of 

 time required by these geological agents to accomplish the results ob- 

 served. Such processes can not, however, differ essentially other than in 

 degree from those observed today. In my opinion, they have never ceased 

 to be active and have only varied in intensity from time to time. It 

 meets all the requirements, therefore, for our present purpose, to con- 

 sider the phenomena now taking place, or since the hot springs and gey- 

 sers were first brought to the attention of the scientific world, about 

 forty years ago. 



Climatic Conditions 



Precipitation of moisture over the plateau and encircling mountains is 

 far heavier than that taking place over the semi-arid regions below. Not 

 only is the rainfall higher for every month of the year, but the tem- 

 perature is correspondingly lower. Four large rivers — the Yellowstone, 



