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



sociated. Such waters, upon reaching the surface, deposit a 

 portion of their burden of silica, as siliceous sinter, forming 

 the white platforms, cones, and mounds which are a character- 

 istic feature of the three geyser regions of the world. This 

 material covers many square miles in the hot spring region 

 of the Yellowstone National Park, occurring in beds of con- 

 siderable thickness, which are of much geological interest, not 

 only as chemical deposits but because they also afford evidence 

 of the age of the hydrothermal forces which have played so 

 important a part in the later history of the region and to which 

 it owes its popular name of " Wonderland." 



Although the formation of siliceous sinter has been noticed 

 by many observers in connection with geyser and hot spring 

 waters in Iceland and elsewhere, it is found that the causes ad- 

 vanced to account for the precipitation of the silica from the 

 hot waters and its deposition as sinter do not offer a satisfac- 

 tory explanation of the origin of many of the deposits of this 

 material found about the hot springs of the Yellowstone. 

 Such deposits are largely, sometimes wholly, due to a separa- 

 tion of silica by the vital growth of the algous vegetation of 

 the hot spring waters. The present article is extracted from a 

 paper in the Ninth Annual Report of the Director of the Survey, 

 in which the work of this vegetable life of thermal waters is 

 fully described. 



The conditions governing the stability of a solution of silica 

 are but imperfectly known, especially for such complex solu- 

 tions as those of the Yellowstone hot springs, however the 

 causes producing a separation of silica from these natural waters 

 can be grouped under the following heads : 

 Relief of pressure. 

 Cooling. 



Chemical reaction. 

 Evaporation. 

 Plant life. 



The first four tend to produce a supersaturated solution 

 of silica, and thus to cause its separation. So far as known 

 the only waters in the Yellowstone Park in which cooling or 

 relief of pressure are operative causes are those of the ISTorris 

 Geyser Basin. If a highly heated underground water dis- 

 solves more silica from the lavas, whose fissures it traverses, 

 than it can retain in permanent solution at the ordinary atmos- 

 pheric pressure and temperature, it becomes supersaturated and 

 deposits silica upon reaching the surface of the ground. The 

 exact amount of silica which can be held in solution by the 

 alkaline waters of the Yellowstone and similar hot springs is 

 not known. The waters most highly charged with silica are 

 those of the Norris Geyser Basin. That of the Opal Spring 



