FLORA OP THE YELLOWSTONE NATIONAL PARK. 9 



Within the Park the Madison River has a drainage area of about 

 700 square miles. 



The region of the Yellowstone Park has been the scene of 

 great volcanic activity, the rocks being mainly of igneous 

 origin with the exception of the Gallatin Range, which is in a 

 great part sedimentary. The plateaus have been formed by 

 great lava flows, principally rhyolite. The rugged Yellowstone 

 Range is mainly composed of volcanic breccias and conglomer- 

 ates. The principal geyser areas are those of the Upper and 

 Lower Geyser Basins, Norris Geyser Basin, Shoshone Lake 

 Geyser Basin and the Heart Lake Geyser Basin. Hundreds of 

 boiling springs are scattered over the whole region and have 

 formed extensive deposits, mainly calcareous, while those of the 

 geysers are a form of silica called geyserite. The great quanti- 

 ties of silicified wood and size of many of the specimens would 

 lead us to suppose that the ancient forests were of much greater 

 magnificence than at present. The wonders of the Yellowstone 

 have been so often described that the above brief sketch will be 

 sufficient for a comparison of the flora with the principal topo- 

 graphic features. 



The Yellowstone Park, like most mountain regions where 

 terrestrial radiation is great, has during the summer months 

 great extremes of diurnal temperature, although the day tem- 

 perature is low, not generally above 75 Fahr. Frequent 

 summer frosts are a characteristic feature. This coolness of 

 climate, coupled with a high relative humidity, that is, for the 

 Rocky Mountain region, accounts for the fact of its being 

 densely timbered. A copious rain-fall is shown in marsh, spring, 

 stream and lake. From observations taken at Yellowstone 

 Lake (7740 feet) from July 15 to August 15, 1885, we obtain 

 the following : Average of readings of minimum thermometer, 

 which practically is the temperature at sunrise, 29'7 ; average 

 of 2 P. M. readings, 65-6 ; average of readings of maximum 

 thermometer from August 1 to August. 15, 68'3. The greatest 

 recorded temperature was 78, and the lowest 22. The grow- 

 ing season over the main area of the Park is from about May 1st 

 to Sept. 1st. The lower and dryer portions up to 7000 feet are 

 in their greenest garb about July 1st, and the subalpine and 

 alpine regions early in August. When in vigorous growth, 



