108 A. HAGUE THERMAL WATERS IN YELLOWSTONE PARK 



strongly contrasted by marked differences in the phases of eruption which 

 built up the two volcanic regions. 



On the rhyolite plateau there are no evidences of violent explosive 

 action. The complete absence of true volcanic breccias is a significant 

 feature of these later flows. Dikes, veins, and horizontal sills, together 

 with nearly all the phenomena of deep-seated intrusions, are wanting. 

 The rhyolite shows scarcely any indications of hydrothermal activity 

 during eruption. In the abrupt escarpments made up of successive 

 sheets there are no signs of surface flows having been exposed to long- 

 continued atmospheric agencies, no wind-strewn ashes, or any vestiges of 

 vegetation. On the contrary, everything clearly indicates a relatively 

 rapid accumulation of viscous masses from the beginning to the end of 

 the rhyolite period. What impresses one most is the absence of stages of 

 activity, with intervals of quiescence, there being rather a series of 

 massive eruptions piled up one on another. In the central portion of the 

 park the rhyolites have a maximum thickness of 2,000 feet, and over 

 large areas they may be assumed to measure 1,500 feet. 



Subsequent to the rhyolites and the building up of the Park Plateau 

 came a few dikes and thin sheets of basalt. They are the most easterly 

 occurrence of those broad basaltic flows that spread over southern Idaho 

 and the Snake River plain. In the park country they are of Pliocene 

 age — that is to say, they are older than the glacial ice. They make the 

 final chapter in the history of Tertiary igneous rocks. As they play no 

 recognized part in the problems bearing on thermal waters, they may be 

 dismissed at the present time with this brief mention. 



Unquestionably the Pleistocene age, with its changed conditions, set 

 in not long after the dying out of rhyolitic eruptions, as is shown by the 

 relatively slight erosion of the plateau and the beginning of canyon 

 sculpturing. All geological evidence tends to prove that the rhyolites 

 belong to the Pliocene age. 



Duration of thermal Activity 



That the activity of thermal waters was approximately coincident with 

 the cessation of rhyolite ejections is, fortunately, clearly proven by the 

 massive horizontal beds of calcium carbonate laid down on the summit of 

 Terrace Mountain, where they attain a maximum thickness of nearly 250 

 feet, although the average is much less. Without doubt they are the 

 oldest deposits of travertine in the region of Mammoth Hot Springs, and 

 rest directly on fresh, unaltered rhyolite. Glacial ice from the Gallatin 

 Mountains moving eastward occupied the intervening Swan Lake Valley 

 and passed over the top of Terrace Mountain on its way to the broad, 



