452 A. Hague — Age of the Igneous Rocks 



following species may be mentioned as characteristic of this 

 horizon : Platanus montana, Quereus yanceyi, Laurinoxylon 

 amethystenium, Populoxylon wardii. 



For the present this group of plant remains has been pro- 

 visionally designated the intermediate flora, as according to 

 Professor Knowlton, it possesses many distinct features of its 

 own, yet appears as a transition flora between that of the acid 

 rocks and the flora of the basic breccia which is of later age. 

 This flora has been referred to the Lower Miocene. 



Directly to the south of the region where both types of rock 

 occur together, the acid breccias suddenly disappear, and later 

 flows bury everything beneath an enormous mass of basic 

 lava. In the deep ravines cut in the abrupt escarpments along 

 the south and west side of Lamar Valley, lies the famous Fos- 

 sil Forest of the Yellowstone Park. The ravines or gorges 

 expose to view vertical sections nearly 2,000 feet in thickness 

 across the bedded agglomerates, mud flows, and basalt sheets of 

 these early basic breecias. From the base of the breccias as 

 exposed along the valley, nearly to their summit on the top of 

 Specimen Ridge the beds carry abundant evidences of an 

 extinct fossil flora. They have so far yielded 70 species, more 

 than one-half being new to science and described by Professor 

 Knowlton for the first time. Those species that are identified 

 as belonging to both the acid and basic breccias are mainly such 

 species as have a wide geological distribution. 



This flora Professor Knowlton regards as distinctly different 

 from that of the acid breccias, and of much later age. In the 

 identification of species and in its affinities it bears a close 

 resemblance to the flora of the Auriferous Gravels of Califor- 

 nia. It has been referred to the Upper Miocene period, and 

 named the Lamar flora. The following species are selected as 

 characteristic of the Lamar horizon : Platanus guillelmaj, 

 Zaurus californica, Magnolia spectabilis, Planera longifo- 

 lia, Magnolia culveri, Aralia whitneyi. 



The flora presents many of the same general characteristics 

 throughout the 2,000 feet of beds, which were derived wholly 

 from eruptive material. It has been found impossible as yet 

 to discriminate between the flora of the lowest beds and those at 

 the top. This would indicate much the same climatic and 

 physical conditions during the entire time required for the 

 accumulation of such a mass of volcanic material with its 

 accompanying flora buried beneath successive layers of tuffs 

 and muds. That there were long periods of rest during the 

 piling up of this material is evident from the great size of the 

 trees, whose trunks are still standing at different elevations in 

 the lavas. 



