404 KEPOKT UNITED STATES GEOLOGICAL SUEVEY. 



Tyndall gives the following explaDation of the manner in which the 

 tube and crater of a geyser are formed: 



Imagine the case of a simple thermal siliceous spring, whose waters trickle down a 

 gentle incline ; the water thus exposed evaporates speedily, and silica is deposited. 

 This deposit gradually elevates the side over which the water passes, until finally the 

 latter has to take another course. The same takes place here, the ground is elevated 

 as before, and the spring has to move forward. Thus it is compelled to travel round 

 and round, discharging its silica and deepening the shaft in which it dwells, until 

 finally, in the course of ages, the simple spring has produced that wonderful apparatus 

 which has so long puzzled and astonished both the traveler and the philosopher.* 



• 



This mode of construction may explain the formation of some cones, but 

 I am inclined to think the number few. A great majority of the basins 

 have no overflow except when the springs are agitated, or when there 

 are bulges of water, and then the overflow is equal in all directions, and, 

 of course, the dejjosition of the silica in such cases is equal on all sides. 

 Where there are waterways leading from the spring, as in the case of 

 the Shield Geyser in the Shoshone Basin and others, there is usually a 

 rim on each side of the outlet, and this is usually continuous with the 

 rim of the spring. During the uplifting of the water, which causes the 

 overfl ow, the water in the basin is usually up to the rim, and it is here 

 that the evaporation causing the deposit takes place, and it goes on as 

 rapidly as in the outlet. In springs that are usually quiet, and where 

 the water stands at one level for a considerable time, the deposit, of 

 course, being at the edge of the spring, there is a constant tendency to- 

 wards narrowing the orifice, and the deposit will either curve towards 

 the center of the spring, as we see in the edge of the beautiful Blue 

 Spring of the Castle Group of the Upper Fire Hole Basin, or a flat edge 

 will be found extending into the spring, as in the Columbia Spring of 

 the Heart Lake Basin. This tendency to the closure of the opening of 

 the spring is seen in the sections shown in the illustrations, Figs. 25 

 and 27. 



To this tendency is due also the approximate conical forms of the 

 craters and the globular form of the inclosed basin. They are all broader 

 at the base than at the top. The water falling on the slope or outer 

 curve, of course, deposits in layers conformable with it. Here we have, 

 perhaps, the explanation of the peculiar form of the Bee Hive. In its 

 eruptions the fall of water is very slight, and the greatest amount of 

 deposition naturally takes place at the top of the cone. The charac- 

 ter of the surface upon which the spring is located has much to do with 

 the form of the mound. Where it is flat, dome-shaped masses will be 

 the result as in the cones in the Sentinel group of the Lower Basin and 

 in the White Dome and the White Pyramid. Where the spring or 

 geyser is situated on a slope a series of terraces will be the result, as in 

 the Te Tarata of oSTew Zealand, and in the slopes of Old Faithful on a 

 small scale. In the latter case the original form was a dome like those 

 now seen in the vicinity. Old Faithful is i3robably of secondary origin, 

 having broken out on the summit of an old mound which may possibly 

 have become choked up. Where the slope is very gentle, as in that 

 surrounding the Great Prismatic Spring of the Egeria Springs, the ter- 

 races are small and flat.t 



Professor Bradley, finding near some of the geysers masses of vol- 

 canic sandstone perforated by irregular holes, which he attributed to 

 the dissolving power of the hot water, thought that it represented, on a 



*From "Heat a Mode of Motion." 



t The lower terraces, at Eotomahana, on the lake shore, are shallower than at the top 

 where the slope is greater. 



