Gleocapsa, a blue-green alga, is found growing on the sides of gey- 

 ser cones, where steam is escaping, forming there a delicate olive- 

 green coloration. A kind of fibrous sinter is formed by the growth 

 of the little alga, Calothrix gypsophila, or the young form of Mastigo- 

 nema therniale, the latter olive colored, and forming the sinter of 

 the crater of the Excelsior Geyser.^ A coarse sinter is due to a 

 bright red species, Leptothrix, a finer variety to Leptothrix i^Hypheo- 

 thrix) lammosa, ranging in color from white to flesh pink, yellow 

 and red to green, as the water cools. Besides the above plants, 

 which belong to the Bacteriace.e and the Cyanophyce^, speak- 

 ing in a general way, we find that several mosses, Musci, are active 

 in the formation of sinter on the slopes below Hillside Spring. 

 These springs issue from the rhyolite slopes beneath the cliffs of 

 the Madison Plateau, and the waters, whose temperatures are 

 184° F.-I98° F., contain both silica and lime in solution, which 

 they deposit in their downward flow. This moss has been deter- 

 mined by Prof. Charles R. Barnes, of the University of Wisconsin, 

 to be Hypmini aduncwn, var. grasilescens, Br. and Sch. 



Besides the sinter and travertine formed by algae, which remove 

 in the case of the carbonated waters, containing calcium bicarbonate, 

 Ca (HC03)2, in solution, the gaseous carbon dioxide, thus depositing 

 calcium carbonate, CaCOg, we have stalactites produced by the 

 growth of several algae, Gleocapsa violacea, Schizothrix calcicola, 

 Synechococcus (Eruginosus and PJiormidiiun {Leptothrix^ laniinoswn. 

 An interesting account of the formation of these stalactites has been 

 given to us by Miss Josephine Tilden, who visited, recently, the 

 Yellowstone Park. 



In the tepid waters of the overflow basins, for example Speci- 

 men Lake, which is produced by the water from the Black Sand 

 Pool, we find extensive diatomaceous beds formed by the growth of 

 numerous diatoms. The water of these areas has encroached on 

 the timber, killing the trees, which stand as bare poles from the 

 treacherous marshes. It is known that these plants deposit silica, 

 as a box, test, or frustule, and it is thus by the activity of the proto- 

 plasm that the silicious diatomaceous earths are formed. Samples 

 of this material show the presence of Denticida valida, which forms 

 the bulk of the material, Denticida elegant, Navicida major, N. viri- 

 dis, Epithema, Cocconema, etc. 



^ Weed, loc. cit. 



