PEALE.] LOWER GEYSER BASIN SENTINEL GROUP. 175 



Ko. 49 is a peculiar-shaped basin 20 by 30 feet. It is somewhat like a 

 boot in shape, and. has a white basin with a white rim, in which the 

 water has a green tint. At one end is a deeper pool which is yellow in 

 color. The temperature was 145° F. 



'No. 50 is a blue, cavern-like spring, with an overhanging white edge, 

 which reaches several feet over the spring. The water is of a handsome 

 bluish- green tint and has a temperature of 165° F. 



As the table shows, only a few of the springs have high temperatures, 

 196° F. (in 1^0.45) being the highest. The highest temperatures are in 

 the bubbling and spurting springs. In 1871 a temperature of 198o F. 

 recorded in was one of the springs of the group. At the lower end of 

 the marsh, situated at the base of a low hill, is a constantly-boiling spring, 

 which has a peculiar toadstool-like rim and sends forth vast columns 

 of steam. In 1871 it had a temperature of 196° F. 



EiaHTH OR SENTINEL GROUP. 



This group of springs was discovered in 1872, and is indicated on the 

 map made in that year as the Eighth Group. Professor Bradley thus 

 describes the discovery : 



From oar camp on tho east side of the Lower Basin we saw on several occasions tall 

 columns of steam rising from near the foot of the ridge on the extreme western side 

 of the basin, hut at first referred them to the cluster among which we had camped 

 on the evening of our arrival. But ui)on examination we found a considerable stream 

 coming from west of the Twin Buttes, which had not been seen by previous explorers, 

 and whose valley included a cluster of large geyser moimds, from Avhich these cohimns 

 of steam must have escaped. Though this group was visited on three different occa- 

 sions, none of us were so fortunate as to witness any eruption from these vents. On 

 entering the valley from below we see before us a range of four large mounds run- 

 ning diagonally across it. The two central ones are the highest, and appear so much 

 as if they were guarding the Upper Valley that this was called Sentinel Branch.* 



Sentinel Creek, as laid down on the map of 1872, is not exactly right, 

 as a comparison with the map accompanying this report will show. 

 We followed it from its mouth and carried a meander line from the 

 Fire Hole to a point above the springs. Just below the springs the 

 valley begins to widen, and is to a great extent a marsh, in which there 

 were probably once numerous springs. The creek flows through it in 

 a deep channel, with muddy banks, which make the crossing difficult, 

 as we found on several occasions. The active period of this basin has 

 I)robably been long passed, and in the isolated mounds we see only the 

 monuments of what were once powerful geysers.t 



It is probable that the only geyser now active in this basin is the one 

 to which I have given the name Eosette. I judge from Professor Brad- 

 ley's description that tbis geyser was not seen by him in 1872. I shall 

 now give the springs of the group in tabular form and then describe 

 specially those that appear to me to be of sufficient importance to de- 

 serve it. 



* Report U. S. Geol. Surv. for 1872, 1873, p. 237. 



t Mr. Norris calls this Basin Geyser Meadows. See his Report for 1881. 



