W. H. Weed — Formation of Siliceous Sinter. 355 



they pass. The larger columnar and vase-shaped forms result 

 from a concurrent life and death of the vegetation, the inner 

 layers dying and decaying as the outer coating of living algae 

 increases. This results in the formation of a hardened bony 

 core, whose innumerable thin layers correspond to successive 

 membranes of algee. A pillar several inches in diameter con- 

 sists of such a bony skeleton surrounded by a thin coating of 

 green or red jelly. When by reason of changing conditions 

 the algse die, a different species of cooler habitat coats the sur- 

 face with a fuzzy nap and adds its quota of silica, to which is 

 added a further coating of silica by the action of the decom- 

 posing vegetation. The granular coating of silica thus formed 

 rounds off and obscures the original outlines of the pillar, now 

 a hard and solid mass of sinter. This conversion of soft algous 

 jelly into stony material is going on in all the cooler pools, 

 where the algous growth has itself dammed back the water, 

 and diverted the supply. Where the basins are filled with 

 algous forms whose tops uniting form a more or less continu- 

 ous roof, supported by innumerable little pillars, the dull gray- 

 ish surface of the sinter, shows no indication of the nature and 

 origin of the sinter beneath, and may serve in turn as the floor 

 of a new basin, and new growth. 



Such is the origin of the sinter deposits of Specimen Lake, the 

 Emerald Spring, north of the Grand Geyser, and numerous other 

 parts of the Upper and Lower Geyser Basins. Similar to this 

 in its origin and nature is the sinter, resulting from the growth 

 of thick cushions of algous jelly, abundant during the past sea- 

 son in the area overflowed by the waters of the Beauty, Soli- 

 tary and other springs, a sinter forming the greater part of the 

 mound of the Solitary. 



Another form of sinter, quite different from those mentioned, 

 but also formed by algous vegetation, is common in all the gey- 

 ser basins. It consists of fibrous layers, T \ to \ inch thick, each 

 layer resembling a thick and short white fur, and formed by 

 the growth of a cedar-red (Calothrix gypsophila), or an olive 

 green (Mastigonema thermale) alga — each formed of, and 

 encrusted with, silica. 



A bright red alga, Leptothrix ochracea, occurring in hot 

 streams at 110° F. to 130° F., forms thatch-like layers of fibrous 

 sinter, resembling interlacing straw. This sinter is very abun- 

 dant about the Excelsior Geyser, and together with the variety 

 last mentioned forms nearly the whole of the sinter platform 

 of the Midway Geysers. A section of this plateau, exposed in 

 the walls of the great pit of Excelsior, is twelve feet thick and 

 shows 24 strata, each one composed of many layers of one variety 

 of sinter. In this thickness of twelve feet, ten feet are formed of 



Am. Jour. Sci.— Third Series, Vol. XXXVII, No. 221.— Mat, 1889. 

 23 



