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SCIENCE. 



[N. S. Vol. IX. No. 221. 



study of the material, and his researches 

 are now in press. For specific determina- 

 tions of these plants I refer you to his mon- 

 ograph. Already over 150 species of plants 

 have been identified. 



The early acid breccias have yielded a 

 terrestrial vegetation regarded as of earlier 

 age than that obtained from the superim- 

 posed lavas. It has furnished a grouping 

 of species so closely allied to the flora found 

 in the Fort Union beds, near the junction of 

 the Yellowstone and Missouri Rivers, that 

 the two floras are regarded as identical 

 in age, and consequently referred to the 

 Eocene period. From these acid breccias 

 eighty species have been identified, and 

 twelve of them were previously only 

 known as belonging to the Fort Union hori- 

 zon. Still others are common to both 

 localities, but are found elsewhere as well. 

 About one-half of the species are new to 

 science, but according to Professor Knowl- 

 ton their biological affinities relate them 

 closely to the Fort Union flora. A second 

 grouping of fossil j)lants, designated for 

 convenience the intermediate flora, flour- 

 ished at a time when the early acid breccia 

 had about ceased to be emitted ; at least 

 thej' occur near the base of the lower basic 

 breccia in beds indicating a mingling of 

 both tj'pes of rook. In all probability they 

 represent a flora which flourished in quies- 

 cent times, during a transition period from 

 •one series of eruptions to another, but fore- 

 shadowing a period of basic eruptions. This 

 flora is of the highest geological signifi- 

 cance, since it indicates a great duration of 

 volcanic activity, with a change of cli- 

 matic and phj'sical conditions. This inter- 

 mediate flora embraces about thirty species, 

 •of which only two or three are as yet known 

 in the acid breccias. About the same num- 

 ber have been recognized as common to the 

 basic breccias, but the evident affinities of 

 the grouping are such that the flora as a 

 whole is apparently more closely allied to 



the overlying than to the underlying rooks. 

 For this reason it is referred to the base of 

 the Miocene period and is regarded as older 

 than the flora of the auriferous gravels of 

 California. 



The vegetation which flourished during 

 the period of the basic breccias was, like 

 the breccias themselves, widely distrib- 

 uted over the mountains wherever mud 

 and silts were present to furnish a suitable 

 soil. No where can it be better studied 

 than at the fossil forest of Specimen Ridge, 

 in the Yellowstone Park, first explored by 

 Professor W. H. Holmes over twenty-five 

 years ago. Since that time other localities 

 have been discovei-ed, and quite recently 

 beds holding leaf impressions of a similar 

 flora have been found on the east side of the 

 mountains. At the fossil forest precipitous 

 walls expose nearly 2,000 feet of horizontal 

 beds of breccias, silts and mud flows, in 

 part laid down by floods and freshets and 

 in part deposited by quiet waters. From 

 base to summit at frequent intervals a ter- 

 restrial vegetation has sprung up and flour- 

 ished, only in turn to be destroyed by re- 

 newed lava streams. In one of these buried 

 forests a stump of a still- standing coniferous 

 tree measures 10 feet in diameter and is 

 surrounded by many fallen logs long since 

 preserved by silicification. If one considers 

 the length of time it takes for any vegeta- 

 tion to spring up on an arid lava field and 

 the great age of many of these trees, the 

 time necessary to build up a series of such 

 forests one above another can hardly be 

 overestimated. That there were long 

 periods of rest between the outpourings of 

 the lava seems evident. Throughout this 

 2,000 feet of erupted material it has been 

 found impossible as yet to discriminate be- 

 tween vegetation found at the base of the 

 cliffs and that interbedded with the lavas 

 at the summit. This implies similar cli- 

 matic conditions during the time demanded 

 to renew and develop a varied flora between. 



