110 A. HAGUE — THERMAL WATERS IN YELLOWSTONE PARit 



Snake, Madison, and Gallatin — carry the waters from the uplands to the 

 lowlands. Knowing the amount of water leaving the park by these 

 principal drainage channels, it is easy to estimate approximately the 

 total amount of surface waters carried away. 



Meteorological records, more or less complete, have been kept at Mam- 

 moth Hot Springs for over a quarter of a century, and during one winter 

 at the Firehole Basin. From these data an approximate estimate can be 

 made of the water falling over the entire region. Some years ago instru- 

 mental measurements were undertaken during the summer to determine 

 the amount of evaporation on the open sinter plain in the Upper Geyser 

 Basin. Similar observations were made at the outlet of Yellowstone 

 Lake. Taking into consideration the annual precipitation and run-off 

 and the summer evaporation, I believe the supply of water greatly ex- 

 ceeds the amount carried away by surface streams. Climatic conditions, 

 as they exist in the park today, favor forest development and a varied 

 undergrowth. It is estimated that over 82 per cent of the region is 

 forest-covered. For eight months precipitation occurs in the form of 

 snow, which, protected by the forests from the sun^s rays and the drying 

 winds, melts slowly and lingers on well into midsummer. On the adjoin- 

 ing mountains the snow seldom entirely disappears. The retention of the 

 water by forest and undergrowth brings about the development of the 

 many meadows, marshes, and bogs. Scattered over the tableland occur 

 frequent ponds and lakelets, carrying in the aggregate a very considerable 

 body of water. In this connection may be mentioned such large reser- 

 voirs as Yellowstone Lake, covering over 125 square miles of surface, 

 and Shoshone Lake, measuring 12 square miles, to say nothing of other 

 picturesque sheets of water of less imposing dimensions, all of which lie 

 on the rhyolite from 500 to 700 feet above the Upper Geyser Basin, 

 where the greatest number of large geysers is found and the activity and 

 overflow of thermal waters displayed on a grand scale. In time much of 

 the water from the meadows and ponds naturally finds its way to surface 

 streams. Another portion is taken up by the luxuriant vegetation or is 

 absorbed by the atmosphere. The remaining water, which constitutes a 

 very considerable volume, is drawn down through openings into under- 

 ground reservoirs. In other words, these descending waters slowly per- 

 colate through the viscous lavas. 



Physical Structure of Ehyolite 



Eeturning for a few brief moments to rhyolite flows, let us consider 

 certain physical features due to textural modifications. No region sur- 



