402 EEPORT UNITED STATES GEOLOGICAL SUEVEY. 



when there is much splashing over the edges, masses will be formed 

 that when broken seem to be composed of bundles of siliceous stems, 

 cemented together. 



In the shallow pools surrounding some of the geysers pebbles are 

 found. Some of these are very regular and very nearly spherical (see d, 

 Plate XLIV), while others are very irregular, some beiug polyhedral. 

 They are frequently covered with small nodules (see a, Plate XLIII). 

 These may be formed in two ways : first, by constant surface accretions 

 to a nucleus, which appears to be proved by the concentric structure of 

 some that have been examined; or, second, as Professor Comstock sug- 

 gests, they may have been worn down from forms with a more or less 

 curved outline. He says : 



These pebbles have therefore been worn down to their present shape by attrition 

 in their pools, and they have no donbt been first formed as ordinary mechanical de- 

 posits, in the form of protuberances over the general surface, or as divisions of a tes- 

 sellated patch, like portions of the platform near the base of Old Faithful Geyser.* 



In many basins there are irregular pieces of geyserite, smooth on their 

 under surfaces, but beaded or covered with prickly points where they 

 are exposed to the action of the water. They are loose in the basins, 

 and may be divisions of a tessellated patch, as Professor Comstock sug- 

 gests. 



Mushroom-like forms are sometimes seen on the edges of the springs, 

 and in the basins small columns (See a, Plate XLV) are found ;t also 

 rosette-like forms, with a connection between them and the bottom of 

 the pool. Professor Bradley, speaking of these, says (see report for 

 1872, p. 240): 



Judging by their general appearance and conditions, these probably originate as 

 fragments of very thin pellicles, formed by evaporation upon the very surface of the 

 pool, and, broken up by the wind, each fragment tends to sink; but some of them 

 escape that fate, and more material accumulates by evaporation upon their wet edges, 

 so that they become basin-shaped and float securely. I cannot account for the basal 

 spires about which a solid pedestal finally accumulates, except by supposing that 

 they have central nuclei of the small fibers of mycelium, which are floating in all these 

 pools in greater or less abundance. 



Some specimens from the edges of the Pearl Geyser, in the Gibbon 

 Basin, are made up of laminse at the base, one of them being yellow and 

 another white. Above these is a layer, irregular in structure, as though 

 the water had fallen in drops while it was being deposited. The upper 

 surface is different again. It is very irregular, of a porcelain-white 

 color, and studded with small nodular points. This specimen, unlike so 

 many of the geyserites, does not change by exposure. Four years after 

 taking it from the spring it is as hard and compact as when collected. 

 Its structure would appear to prove that the spring has had periods of 

 very different sorts of action. The laminae at the bottom indicate a sim- 

 ple overflow from the basin. This overflow may have been due to sim- 

 ple risings and fallings of the water in the basin. Then there appears 

 to have succeeded a time when the water was in constant agitation, 

 splashing over the sides. This, still later, was followed by a period of 

 spouting, its present condition. These remarks might be extended al- 

 most indefinitely, but enough has been said to show that the forms are 

 almost innumerable in their variety. 



* Reconnaissance of Northwestern Wyoming, &c., by Capt. W. A. Jones, 1873, p. 258. 



t Similar small columns are seen in springs at Rotomahana, in New Zealand, and 

 the Rev. R. Abbay (Jour. Geol. Soc, Vol. XXXIV, 1878, No. 34, p. 170-178) explains 

 their formation as' follows: He thinks the water is saturated with silica and has dif- 

 ferent temperatures. The water coming from two different basins meeting, a condensa- 

 tion occurs analogous to what occurs in the atmosphere when two masses of air, in 

 "which the temperatures are different, come together. 



