xlvi GENEKAL SmiMAllY OF SCIENTIFIC AND 



laceous strata, the latter Avitli lignites and coal. Overlying 

 these, in a nearly horizontal attitude, are fresh-water sand- 

 stones, conglomerates, and marls, probably of Upper Eocene 

 age. Of the volcanic rocks of the region, a hornblendic tra- 

 chyte is said to overlie the Cretaceous, while there are out- 

 bursts of much earlier date, and others of Post-pliocene age. 

 The volcanic activity of the great plains of the Snake Riv- 

 er basin was also, according to Bradley, comparatively re- 

 cent, and the movements of the surface were still later, as is 

 shown by the fact that basalts and porphyries, interstratified 

 with Pliocene sandstones and limestones, are found upheaved 

 at the base of the foot-hills. The boiling springs in the basin 

 of the Upper Madison River rise from sandstones which ap- 

 pear to have been deposited in a volcanic crater of large di- 

 mensions, and the subterranean fire still heats the Avater 

 Avhich sinks into these porous beds, causing it to I'ise again 

 heated to a point above the boiling-point at this altitude 

 (about 200 Fahr.). The sandstones are perforated and eroded 

 by the solvent power of these heated waters, which dissolve 

 the silica to deposit it again in the vicinity as the water 

 cools. Gelatinous vegetable forms grow in all these pools 

 where the water is not in such violent ebullition as to break 

 them up and destroy them. Tliey are sometimes broad, thick 

 sheets or branching sponge-like forms, green or rusty-brown 

 in color, and sometimes white and fibrous. The latter, which 

 grow most abundantly in the rapidly flowing outlets of the 

 pools, are constantly incrusted with silica, and as constantly 

 reproduced. The larva? oi Ilellcopsyche were also met with 

 in a pool at 180 Fahr., and living diatoms in water at over 

 100 Fahr. The deposited silica has in some parts cemented 

 sand and gravel beds into hard conglomerates, and even into 

 perfect quartzites, and the silicification of Avood in all its 

 stages is seen in the pools. In this connection we may notice 

 the investigations by John Arthur Phillips of the well-known 

 silicified woods from the auriferous gravels of California, 

 probably, according to Newberry, of later Pliocene age. Some 

 of the trunks were silicified without previous change, while 

 otiiers were first converted more or less completely into lig- 

 nite before silicification. In the latter case an amount of 

 carbonaceous matter, equal to about fourteen per cent., is 

 still preserved. The replacing silica, nearly pure and slight- 



