DEVELOPMENT OF SPRINGS AND GEYSERS 119 



at the present time. Geologically speaking, the final stage of thermal 

 activity is a hot spring. The tendency of a geyser is to develop a hot 

 surface pool. If from such pools there is discharged a sufficient amount 

 of overflow, and if from the surface of these geyser pools there is an 

 ample dissipation of heat into the surrounding atmosphere, explosive 

 action may cease and the geyser, as such, may become extinct. It is fre- 

 quently stated that some geyser has ceased to be active, and that this 

 indicates the slow dissipation of the original source of heat. This I 

 believe to be an error. The change is simply due to a shifting of the 

 channel of the ascending waters. 



If, on the other hand, there should be marked climatic changes and 

 arid conditions should set in over the park and adjoining mountains, in 

 my opinion, thermal springs would become extinct. Should this happen 

 it would be evident beyond all question that the waters were derived 

 from vadose sources. Again, with the disintegration of lavas and the 

 building up and enlargement of reservoirs, existing conditions of hydro- 

 static pressure would cease and the circulating waters, unable to rise, 

 would distribute themselves laterally; in which case there might break 

 out at the base of the rhyolite plateau calcic springs such as we now find 

 and have already described. 



In all probability the magnitude of a geyser is, in a measure, depend- 

 ent on the size of the underground reservoir, or series of reservoirs, pro- 

 duced by the disintegration of lavas along channels of ascending waters. 

 It has been demonstrated by self-registering thermometers that cool infil- 

 trating waters may drain into partially erupted reservoirs after geyser 

 eruption. This has been shown in the case of the Giantess Geyser. The 

 question was once asked by an attendant at the hotel, who had spent 

 several summers in the park, why Old Faithful was more apt to be sev- 

 eral minutes behind time in September than in July. I am not aware 

 that such a condition was ever established, but if so, my reply would be 

 that in the autumn the infiltration of surface waters is not as rapid as in 

 early summer; hence a retarding of the eruption by several minutes. 



It is probable that the Norris Geyser Basin, in its thermal develop- 

 ment, is later than the Upper Geyser Basin. In the former are found 

 the early and more acid conditions ; the waters of the geysers are mainly 

 neutral and form deposits of arsenical sulphides, alum, and ferric salts. 

 These phenomena are for the most part absent in the Upper Geyser Basin, 

 where the waters have reached a more advanced stage and possess a 

 siliceous alkaline composition. 



With the exception of arsenic and boron, which occur in minute quan- 

 tities, all the elements brought to the surface in solution by the thermal 

 waters of the park have been found in the rhyolite. In this connection 



