356 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



is surrounded on all sides by steam and dense vapor. The broad cir- 

 cular mound Burrounding the orifice of the geyser is built up by a series 



of low sinter terraces, many of them holding pools into which the 

 descending water falls. The great regularity of Old Faithful makes it 

 an object of intense interest to tourists and one seen by all visitors to 

 the Park. 



LOWER GEVSKK BASIN TO YELLOWSTONE LAKE. 



Returning to the Lower Geyser basin the road turns off to the east 

 and follows Nez Perce valley for nearly 10 miles. On entering the valley 

 the Morning Mist group of springs is seen on the south side of the road, 

 skirting the base of the hills and extending up on to the ridge. The 

 hillsides are covered with small springs and steam vents, concealed by 

 timber during the greater part of the day. but in early morning pre- 

 senting a picturesque appearance by the numerous columns of steam 

 which rise through the dark green forest. The Nez Perce" is character- 

 istic of the larger valleys of tin 1 region, and evidences of its former 

 occupation by ice may be seen on the long monotonous lava ridges. 



A steep ascent of 900 feet leads up to the top of the central plateau, 

 and from an opening in the timber on the edge of a steep cliff a magnifi- 

 cent view may be had looking backward down the valley and out over 

 the geyser basins and Madison plateau beyond. The road then crosses 

 a treeless portion of the plateau over pearlite, obsidian, and various 

 modifications of glassy rhyolite. 



The Highland Springs area lies just south of the road on the very top 

 of the plateau. It is a region desolate beyond description, but of great 

 interest in a study of the action of acid waters upon siliceous lavas. 

 The waters of Alum creek, which rise in the Highland springs, have a 

 very obnoxious and astringent taste. 



Leaving the plateau, a descent of 200 or 300 feet brings us out into 

 Wayden valley, a broad, shallow depression in the rhyolite. The 

 underlying rocks are modifications of glassy rhyolite, over which occur 

 Tertiary lake beds and morainal material of the glacial period. 



After skirting the southern end of the valley along the edge of the 

 timber, the road turns southward, following Yellowstone river for eight 

 miles to the Lake. Shortly after leaving the valley a hot spring area 

 several acres in extent is reached, extending from the banks of the river 

 up on to the slopes of the ridge. It is quite like other areas in most of 

 its thermal manifestations, but the Mud geyser and the Mud volcano 

 have attracted more than ordinary interest. The Mud geyser behaves 

 like an ordinary geyser, except that instead of ejecting clear water it 

 contains a mixture of clay and water, throwing the slimy material from 

 20 to 30 feet in the air. The Mud volcano is situated a short distance 

 from the geyser on the steep side of a hill. A caldron 20 feet in depth 



