142 GEOLOGICAL HISTORY OF THE YELLOWSTONE PARK. 



iiig". This is not without interest, as evidence of uudei-gTouud heat 

 may be observed everywhere throughout the Park in the waters of the 

 geysers and hot springs. All our observations point in one direction 

 and lead to the theory that the cause of the high temperatures of these 

 waters must be found in the heated rocks below, and that the origin of 

 the heat is in some way associated with the source of volcanic energy. It 

 by no means follows that the waters themselves are derived from any 

 deep-seated source; on the contrary, investigation tends to show that 

 the waters brought up by the geysers and hot springs are mainly sur- 

 face waters which have percolated downward a sufficient distance to 

 become heated by large volumes of steam ascending through fissures 

 and vents from much greater depths. If this theory is the correct one 

 it is but fair to demand that evidence of long-continued action of hot 

 waters and super-heated steam should be apparent upon the rocks 

 through which they passed on their way to the surface. This is pre- 

 cisely what one sees in innumerable places on the Park plateau. In- 

 deed, the decomposition of the lavas of the rhyolite plateau have pro- 

 ceeded on a most gigantic scale, and could only have taken place after 

 the lapse of an enormous period of time and the giving off of vast 

 quantities of heat, if we are to judge at all by what we see going on 

 around us to-day. The ascending currents of steam and hot water 

 have been powerful geological agents, and have left an indelible im- 

 pression upon the surface -of the county. The most striking example 

 of this action is found in the Grand Canon of the Yellowstone. From 

 the lower falls for o miles down the river abrupt walls upon both 

 sides of the canon, a thousand feet in de]Dth, pri'seut a brilliancy and 

 mingling of color beyond the power of description. From the brink of 

 the canon to the water's edge the walls are sheer bodies of decomposed 

 rhyolite. Varied hues of orange, red, purple, and sulphur-yellow are 

 irregularly blended in one confused mass. There is scarcely a piece of 

 unaltered rock in place. Much of it is changed into kaolin; but from 

 rhyolite, still easily recognized, occur tiansition products of every pos- 

 sible kind to good jjrocelain clay. This is the result of the long- con- 

 tinued action of steam and vapors upon the rhyolite lavas. Through 

 this nuiss of decomposed rhyolite the course of ancient steam vents in 

 their upward passage may still be traced, while at the bottom of the 

 canon hot springs, fumaroles, and steam vents are still more or less 

 active, but probably with diminished power. 



It is needless to weary you with the details of this decomposition, 

 but I may add that investigations in the laboratory upon these tran- 

 sition products fully substantiate field observations. 



Still other areas are quite as convincing, if not on so grand a scale, 

 as the Yellowstone Canon. Joseph's Coat Basin, on the east side of 

 the canon, and Brimstone Hills, on tin; east side of the Yellowstone 

 Lake, an extensive area on the slopes of the Absaroka Eange, both 

 present evidences of the same chemical processes brought about in the 



