GEOLOGICAL SURVEY OF THE TERRITORIES. 235 



Here, I think, we have an example, on a small scale, of the process by 

 which the large cavernous pools, of which so many examples occur all 

 about us, have been excavated. It would appear that, in many cases 

 at least, the hot alkaline (?) water, as it reaches the surface, is not thor- 

 oughly saturated with silica, and accordingly dissolves away the walls 

 of its orifices, so as to undermine the surrounding area, the surface of 

 which falls in, until, in many cases, large caverns are excavated. This 

 action appears to be limited in two ways : first, the pool may reach such 

 dimensions that the quantity of water constantly supplied bears so 

 small a ratio to the whole contents that the whole pool becomes par- 

 tially cooled as well as somewhat concentrated by evaporation, and so 

 the excavation is checked, if not stopped ; and, secondly, the supply- 

 pipe may become almost entirely stopped up, so that- barely enough 

 water is supplied to make good the loss by evaporation, in which case 

 the solution of silica would become supersaturated, and the silica itself 

 be deposited on the walls and bottom of the pool until it be ultimately 

 closed again, unless the stoppage of the tube be removed, and 

 the succession of conditions thus begun again. If, however, the 

 stoppage of the tube be complete, the silica will soon all be de- 

 posited by reason of the evaporation of the water, and the pool will 

 become a dry cavern, except in positions where it may be kept full by 

 the drainage of surface-waters. The large deep- pools, however, are 

 generally very hot, showing a constant supply from beneath, and many 

 of them are in constant ebullition, though rarely true geysers; indeed, 

 I believe that the Giantess, in the Upper Basin, is the sole exception to 

 the rule, so far as yet observed ; and this is probably in its last stages 

 and near extinction, if we may judge by the length and irregularity of 

 its intervals. 



The smaller vents vary greatly in temperature, depending apparently 

 upon the greater or less freedom of their connection with the heat- 

 centers below. In some cases they are few and solitary, with little or 

 no deposit about them, as if they had been but recently opened. Again, 

 they are in large clusters, of which the more elevated vents are generally, 

 i^hough not always, the hottest. It would appear that the comparatively 

 compact deposit of nearly pure silica, formed by the flowing water, is 

 much less readily dissolved and disintegrated, by the hot flow beneath, 

 than is the silica of the more porous and partly argillaceous volcanic 

 rock which forms the slopes of the hills ; so that, when a vent is once 

 stopped, it is very rarely re-opened, unless by earthquake-cracking, and 

 the checked water is forced backward and escapes at higher and 

 higher points on the hill-sides. In the geysers, the tube, being exposed 

 to the air at frequent intervals, while it is yet dripping with the rapidly 

 evaporating water of previous eruptions, receives constantly new layers 

 of silica, which are but slightly, if at all, re-dissolved by the next erup- 

 tion ; and so its size is constantly decreased, until the tube becomes en- 

 tirely closed and a new vent is opened elsewhere. 



In the clusters of small vents, there is generally more or less sympa- 

 thetic action, showing that they open into some common chamber be- 

 low, but that the connecting passages are more or less contracted, so 

 that the flow of steam and water is far from being controlled by the ordi- 

 nary laws of hydrostatics. Frequently a large vent, whether a geyser 

 or simply a boiling pool, is surrounded by a number of small ones, which 

 are active while it is quiet and quiet while it is active. I stood, one 

 morning, upon the mound of Fountain Geyser, in the Lower Basin, 

 whose pool was filled to overflowing, and was watjching a vehement 

 steam-jet, a hundred yards away, on the lower slopes of the terrace. 



