HAGUE.) 



THE YELLOWSTONE TARK. 343 



may be traced upon both sides of the canyon wall, but not a fragment of 

 rock more than a lew inches in diameter, older than rhyolites, has been 

 recognized within a radius of many miles. This massive block, although 

 irregular in shape and somewhat pointed towards the top, measures 

 24 feet in length by 20 feet in breadth and stands 18 feet above the 

 base. The nearest point from which it could have been transported is 

 distant 30 or 40 miles. Coming upon it in the solitude of the forest, 

 with all its strange surroundings, it tells a most impressive story. In 

 no place are the evidences of frost and fire brought so forcibly together 

 as in the Yellowstone National Park. 



Since the close of the glacial period no geological events have brought 

 about any great changes in the physical features of the region other 

 than those produced by the action of steam and thermal waters. Evi- 

 dences of fresh lava Hows within recent time are wholly wanting. 

 Nevertheless, over the Park plateau the most unmistakable evidence of 

 underground heat is everywhere to be seen in the waters of innumer- 

 able hot springs, geysers, and solfataras. A careful study of all the 

 phenomena leads to the theory that the cause of high temperatures of 

 these waters is to be found in the heated rocks below, and that the 

 originofthe heat is in some way associated with the sources of volcanic 

 energy. Surface waters in percolating downwards have become heated 

 by relatively small quantities of steam rising through fissures from 

 much greater depths, (leysers and springs return these meteoric 

 waters to the surface. Thermal springs, geysers, and solfataras are in 

 a sense volcanic phenomena, and remain as evidence of the gradual 

 dying out of volcanic energy. If this theory is correct, proof of the 

 long-continued action of thermal waters upon the rocks should be 

 apparent, as it is fair to suppose that they must have been active 

 forces ever since the cessation of volcanic eruptions. This is precisely 

 what one may Bee all over the rhyolite area. Ascending currents of 

 Steam and acid waters have acted as powerful geological agents in rock 

 decomposition and have left an indelible impression upon the surface 

 of the country. Large areas of decomposed rhyolite and extinct sol- 

 fataras show the former existence of still greater thermal activity. 

 Rock decomposition and deposition of sediment from siliceous waters 

 are extremely slow processes, if we may judge from what we see going 

 on to day in the different geyser basins. It is evident that to accomplish 

 such changes a long period of time must have been required. 



An evidence <>f the antiquity of the hot spring deposits is shown in 

 an equally striking manner, and by a wholly different process of geo- 

 logical reasoning. Terrace mountain is an outlying ridge of the rhy- 

 olite plateau, just west of the Mammoth Hot springs. It is covered on 

 the Summit with thick beds of travertine, among the oldest portions 

 of the Mammoth Hot springs deposits. It is the mode of occurrence 



