GEOLOGICAL SURVEY OF THE TERRITORIES. 237 



by water falling from the geyser-jets, has prevented their being cemented 

 to the bottom, and has at the same time aided in keeping them smooth. 

 Where these are few, so as to hav^e plenty of room for rolling, they are 

 commonly quite regular, and often nearly spherical ; but, where they 

 are numerous and crowded, they wear each other quite irregularly, so that 

 polyhedral forms abound. Often, while the lower portions are thus poly- 

 hedral, the exposed upper surfaces are quite regularly spheroidal. The 

 original centers of concretion seem to hate been very minute fragments, 

 broken from the surfaces of the geyser-cones, probably by the rush of 

 the eruption. In some cases, the upper surfaces of these concretions are 

 pearly, in others they are beaded ; occasionally the whole surface is beaded, 

 or is covered with prickly points. These surfaces are also frequently 

 deposited upon fragments of the laminated material which may have 

 by any means found their way into these crystallizing baths. These 

 pearly and beaded and prickly surfaces are also individual characteris- 

 tics of the deposits of different geysers, and vary greatly in detail. The 

 immediate orifice of a geyser is almost universally beaded, and this 

 character extends to greater or less distances from it, according to the 

 distribution of the falling water, surfaces frequently thus washed or 

 sprinkled being almost universally of this character. Such surfaces, on 

 the contrary, as are frequently bathed in steam, without much spray, 

 are nearly always pearly, as if the steam itself carried enough silica to 

 form the extremely thin layers which are essential to pearly luster. 

 This is commonly the character of the upper surfaces of those coral-like 

 growths which form along the borders of many of the quiet hot pools, 

 while their lower surfaces are covered with prickly points. In nearly 

 every pool, except where ebullition is so strong as to break np such ten- 

 der tissues, we see gelatinous vegetable forms allied to mycelium, or 

 the "mother" of vinegar, sometimes in broad, thick sheets, sometimes 

 in thick branching forms, resembling sponges, sometimes in long waving 

 fibers. The former kinds are generally either green or rusty brown ; 

 the fibrous forms are generally pure white. These are very common in 

 the rapidly-flowing outlets of the hot pools, and are continually repro- 

 duced as these channels fill up with newly-deposited silica ; so that, in 

 breaking through the crust, we often find laminae filled with molds of 

 those fibers, and sometimes to such an extent as to closely resemble 

 silicified wood. 



From our camp on the 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, but at first referred them to 

 the cluster among which we had camped on the evening of our arrival. 

 But upon 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-mounds, from which these 

 columns of steam must have escaped. Though this group was visited 

 on three different occasions, 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 running 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. 

 Approaching this row from the east, the first spring observed is upon a 

 low mound, near the foot of the spur which separates this valley from 

 the main one. It is a deep oval pool, whose two diameters are about 

 12 and 15 feet, with a beautifully-scalloped edge, not raised above the 

 general surface of the mound. The temperature is 198°, and this is 

 probably a geyser. Several small vents also appear on this mound, but 



