3G2 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



size and. becomes an Opaque white mass of silica, whose weight is not 

 one per cent of its former state. Chemical analysis shows this dried 

 material to be silica and water, viz: 



si0 - 93. 87 



«*° 1.17 



Organic matter i. -,o 



Experiments showed the writer thai (he growing algaa form a jelly 

 of hydrous silica; it is of this material that the alga aiaments are 

 formed, and the algse slime of other waters is here a hydrous silica 

 binding the threads together. The nature of this separation may 

 be seen under the microscope, thongh the fresh hydrous silica is diffi- 

 cult to study, and the dried material becomes opaqne. In most cases 

 the glassy rods can be readily distinguished and the inclosing paste 

 usually shows globules and pellets of the dehydrated silica. 



The process of sinter formation is best illustrated, and its importance 

 most apt to be appreciated, by an examination of an area covered by a 

 large and constant overflow, such as that from the Black Sand whose 

 connected overflow pools are known as Specimen lake. Here the 

 alga> glowing in the wafers rapidly choke up the channel and cause 

 the main supply to be diverted. Basins are formed by the algous 

 growths, and in them pillars grow op from the bottom that are often a 

 foot in height. These increasing in number finally fill the pool, their 

 tops reaching the surface coalesce and roof over the basin until the 

 waters, becoming choked, Beek other outlets. The gradual lessening of 

 this supply of water causes the final death of the algae. In the cool 

 waters that till the space between the pillars the hydrous silica begins 

 to harden. Aided by the acids of the decomposing vegetable matter 

 this process is quite rapid, and more silica is separated from the cold 

 water to form a coral-like coating, and finally the former soft algous 

 jelly becomes a hard and firm rock. Eventually diversion of the hot 

 waters builds up another growth upon the old one and thus the chan- 

 nel, swinging around from side to side, successively forms new basins, 

 new growths, and new deposits of silica. 



Every step of this process has been patiently studied for many years. 

 In the majority of cases it is less easy to recognize than at Specimen 

 lake. In the channels that carry off the water ejected from Old Eaith- 

 ful geyser, for instance, a different species of algffl from that building 

 up the deposits of Specimen lake forms a velvety furze upon the chan- 

 nel floor. Its color is a brilliant orange to a cedar-red or dark seal- 

 brown, depending upon the temperature. This species, identified as 

 Calothrix by Wollc, forms a delicately fibrous but close-grained sinter, 

 quite unlike the coralline masses of Specimen lake. The tangled silky 

 skeins seen floating in the overflow of the Giantess and other geysers, 



