354 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



On the west Bide of the river, between the two large geyser basins, 

 lies a small but interesting geyser region known as Midway basin. It 

 is a dazzling white sinter plain, without tree or meadow, the only relief 



to the eye being enormous volumes of steam rising from hot lakes and 

 cauldrons. This basin contains the Excelsior, the grandest and most 

 imposing geyser, and Prismatic lake, a singularly beautiful sheet of 

 water unsurpassed for brilliancy of color and exquisite beauty of its 

 rim. Excelsior geyser throws an enormous column of water, the more 

 powerful eruptions emitting a stream 260 feet into the air, measuring 

 20 feet in diameter at the base and breaking into a fan-shaped body 

 above. It rises from a circular cauldron of boiling, steaming water, 

 the level of which lies about 15 feet below the surface of the sinter 

 plain. The cauldron wall affords an excellent opportunity for a study 

 of sinter deposition. The amount of water thrown out during any vio- 

 lent explosion reaches many thousand barrels, which, pouring over the 

 edge ofthe cauldron, runs rapidly across the sloping plain and down 

 the banks of the river into the stream below. The level ofthe Firehole 

 river is frequently raised several inches, the water showing a marked 

 increase in temperature for a long distance below the geyser. Fre- 

 quently large blocks of sinter are hurled violently into the air by the 

 force of the explosion. 



The bluff on the east side of the Firehole river, opposite Excelsior 

 geyser, is rhyolitic pearlite, and from the top of the cliff, on a clear day 

 and the wind westward, Prismatic lake may be seen to great advan- 

 tage. 



From Midway basin the road passes a number of hot springs, but 

 none of special interest until reaching the Sapphire group, situated on 

 the west side of the Firehole. It contains a number of small geysers, 

 each exhibiting some novel feature of thermal action peculiar to itself. 

 Sapphire pool, a large circular basin raised slightly above the sinter 

 plain, has scarcely any rival among the marvelous springs along the 

 Firehole. It closely resembles the Great Fountain in the character of 

 its sinter deposits and overflow basin. The water is of the deepest 

 blue and the temperature always stands near the point of ebullition. 

 Near by are the .Jewel, Silver Globe, and Avoca. 



For a mile along the river, before reaching the Upper Geyser basin 

 proper, there is a line of hot springs and geysers, indicating a great 

 amount of thermal action along the valley. Among the most important 

 may be mentioned the Cauliflower, (Jem, Artemisia, and Morning Glory. 

 A good illustration ofthe difficulty which may sometime arise in distin- 

 guishing a hot spring from a geyser is seen in the case of the Artemisia, 

 which for a long time was supposed to be a quiet pool, but in recent 

 years has exhibited all the phenomena of explosive geyser action. 



