558 



SCIENCE 



[N. S. Vol. XXXIII. No. 850 



Following the withdrawal of a broad ice 

 sheet, ascending heated waters, acting with 

 renewed energy upon the walls of in- 

 numerable fissures and rifts, bleached and 

 kaolinized massive blocks of rock. This 

 decomposition of plateau lavas proceeded 

 on a grand scale and left an indelible im- 

 pression upon the rhyolite areas. In re- 

 gard to the age of the hot springs it is 

 reasonable to conclude that thermal waters 

 were as active at the close of the rhyolite 

 extrusions as at any subsequent period. 

 The antiquity of many localities of decom- 

 posed rhyolite is clearly evident, as shown 

 by post-glacial sculpturing. In certain 

 areas where hydrothermal energy was 

 formerly a long-continued process, evi- 

 dence of the presence of such sources of 

 heat have long since ceased. No one who 

 has studied the gradual development of 

 these decompositions and metasomatic 

 changes under the influence of acid sol- 

 fataras, or the deposition of sinter now 

 taking place from alkaline siliceous waters, 

 can doubt the lapse of time required by 

 these geological agents to accomplish the 

 results observed. Such processes can not, 

 however, differ essentially other than in 

 degree from those observed to-day. In my 

 opinion, they have never ceased to be active 

 and have only varied in intensity from 

 time to time. It meets all the requirements, 

 therefore, for our present purpose, to con- 

 sider the phenomena now taking place or 

 since the hot springs and geysers were first 

 brought to the attention of the scientific 

 world, about forty years ago. 



CLIMATIC CONDITIONS 



Precipitation of moisture over the 

 plateau and encircling mountains is far 

 heavier than that taking place over the 

 semi-arid regions below. Not only is the 

 rainfall higher for every month of the 

 year, but the temperature is correspond- 



ingly lower. Four large rivers, the Yel- 

 lowstone, Snake, Madison, and Gallatin, 

 carry the waters from the uplands to the 

 lowlands. Knowing the amount of water 

 leaving the park by these principal drain- 

 age channels, it is easy to estimate ap- 

 proximately the total amount of surface 

 waters carried away. 



Meteorological records, more or less com- 

 plete, have been kept at Mammoth 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 in- 

 strumental 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 considera- 

 tion the annual precipitation and run-off, 

 and the summer evaporation, I believe the 

 supply of water greatly exceeds the 

 amount carried away by surface streams. 

 Climatic conditions, as they exist in the 

 park to-day, 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, pro- 

 tected by the forests from the sun's rays 

 and the drying winds, melts slowly and 

 lingers on well into midsummer. On the 

 adjoining mountains the snow seldom en- 

 tirely 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 

 reservoirs as Yellowstone Lake, covering 

 over 125 square miles of surface, and She- 



