GEOLOGICAL HISTORY OF THE YELL0W8T0NE PARK. 143 



saiiu' luaniier. It is not statiiij;- it too stioiigly to say tliat the plateau on 

 the east side of the (irand Canon, from Uroad (Jreeiv to Telican Creek, 

 is eoni])letely undermined by the action of superheated steam and 

 alkaline waters on the rhyolite lava. Simihir processes may be seen 

 ijoing- on to-day in all the geyser basins. To accomplish these changes 

 a long period of time must have been recpiired. The study of com- 

 paratixely fresii vents shows almost no change from year to year, al- 

 though careful scrutiny during a period of tive years detects a certain 

 amount of disintegration, bnt infinitely small in comparison with the 

 great bo<lies of altered rock. This is well shown in a locality like the 

 Monarch Geyser in the iSTorris Geyser Basin, where the water is thrown 

 out at regular intervals through a narrow fissure in the rock. 



The Grand Canon of the Yellowstone otters one of the most impres- 

 sive examples of erosion on a grand scale within recent geological 

 times. It is self-evident that the deep canon must be of much later 

 origin than the rock through whicih it has been worn, and it seems 

 (piite clear that the course and outlines of the canon were in great part 

 determined by the easily eroded decomposed material forming the 

 canon walls, and this in turn was brought about by the slow processes 

 just described. 



The evidence of the anti(iuity of the hot spring deposits is, perhaps, 

 showji in an equally striking manner and by a wholly ditterent proc- 

 ess of geological reasoning. Terrace Mountain is an outlying ridge of 

 the rhyohte plateau just west of the Mammoth Hot S])rings. It is 

 covered on the summit with thick beds of travertine, among the eldest 

 portions of the Mammoth Hot Springs deposits. It is the mode of 

 occurrence of these calcareous deposits from the hot waters which 

 has given the name to the mountain. Lying upon the surface of 

 this travertine on the top of the mountain are found glacial bowlders 

 brought from the summit of the Gallatin Range, fifteen miles away, 

 and transported on the ice sheet across Swan Valley and deposited 

 on the top of the mountain, TOO ieet above the intervening valley. It 

 offers the strongest possible evidence that the traveitine is older than 

 the glacier which has strewn the country with transported material. 

 How much travertine was eroded by the ice is, of course, impossible to 

 say, bnt so friable a material would yield readily to glacial movement. 



Still another method of arriving at the great anti(iuit>- of the thermal 

 energy and the age of the hot spring foimation is by determining the 

 rate of deposition and measuring the thickness of the accumulated 

 sinter. This method, although the one which would perhaps first sug- 

 gest itself, is in my opinion by no means as satisfactory as the geo- 

 logical reasoning already given. It is unsatisfactory because no uni- 

 form rate of dei)osition can be ascertained for even a single area, like 

 the r))])er Geyser Basin, audit is still more dilficult to arrive at any 

 conclusion as to the growth of the sinter in the past. Moreover, it is 

 quite possible that heavy deposits may have suftered erosion before the 



