SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1919. 



Denver Basin, wliich yields the flora of the 

 Denver formation. 



The tuffs containing this flora are thus 

 about halfway between the Gulf Coastal Plain 

 province and the rocks containing the south- 

 ernmost known early Eocene floras along the 

 present Rocky Mountain front. The flora is 

 therefore of great mterest for the information 

 which its study affords regardmg former cli- 

 matic conditions in the region of its occurrence. 

 Unfortimately the number of plants described 

 is entirely too small to furnish precise paleo- 

 ecologic data. At the same time certain lead- 

 ing features stand out very clearly despite the 

 small number of forms represented. Baker 

 and Bowman report an abmidance of silicified 

 logs from the tuffs, some as large as 3 feet in 

 diameter. I have specimens of at least two 

 species of petrified wood, which have not yet 

 been sectioned, but one is of an open-pored, 

 dicotyledonous type and indicates climatic 

 conditions quite different from the semiaridity 

 that now prevails in trans-Pecos Texas. 



All the identified forms from these tuffs, 

 except the two new species, are common to the 

 floras of either the Midway (?) or Wilcox of 

 the Coastal Plain or to the floras of the Raton 

 or Denver of the Rocky Mountain province. 

 This in itself is conclusive proof that the gen- 

 eral climatic conditions in trans-Pecos Texas 

 at that time were very sunilar to those of the 

 coastal region to the southeast and of the in- 

 terior region to the northwest. It also fur- 

 nishes collateral evidence of the lack of a lofty 

 mountain axis traversing the western interior 

 at that time and shows that an interchange of 

 life forms between the southeastern coastal 

 region and the interior region was readily pos- 

 sible — a conclusion sustained by the consider- 

 able number of species known to be common to 

 the early Tertiary floras of the GuK Coastal 

 Plain and those of the West (Denver, Raton, 

 Fort Union, etc.). 



The presence of two individually abundant 

 species of palms in the flora from the Barilla 

 Mountains, one a fan palm and the other a 

 pinnately veined palm of the feather palm 

 type, may be noted. The fan palm {Sabalites) 

 appears to be most like the existing species of 

 Sahal, the arborescent forms of which are now 

 essentially coastal types that extend as far 

 northward as latitude 35°. The botanic affin- 

 ity of the fossil is, however, not certainly de- 



terminable, so that too much reliance can not 

 be jilaced on the temperature and moisture 

 requirements of the existing cabbage palmetto. 

 The other palm (Geonomites) represents a 

 group of fossil forms typified in the existing 

 flora by the genus Geonoma. Geonoma is now 

 widely distributed in tropical and subtropical 

 America and finds its maximum representation 

 in the Amazon River basin. The fossil forms 

 that have hitherto been referred to Geonormtes 

 are found in assemblages which indicate a 

 range from humid warm temperate conditions 

 (for example, those of the Denver flora) to 

 what appear to have been typically tropical 

 conditions (for example, those of the flora of 

 the Italian lower Oligocene). Thus the pres- 

 ence of a small Geonomites in the Barilla 

 Mountain region argues for an abundant 

 supply of moisture, like that required by the 

 Sabalites, and for a somewhat warmer tem- 

 perature. The true measures for both tempera- 

 ture and moisture of this region in Eocene time 

 may properly be considered as falling some- 

 where between the extremes indicated for 

 these two types of palms. 



The Juglans, from its distribution during the 

 Eocene as well as its general geologic history 

 and the existing ranges of its modern repre- 

 sentatives, may be taken to indicate warm 

 temperate conditions of abundant but not 

 excessive precipitation. Tlie Asimina and 

 Oreodaphne fit in nicely with such assumed 

 conditions, for while both genera have tropical 

 representatives, others of their species extend 

 long distances into the Temperate Zone, as for 

 example Asimina triloba Dunal, our common 

 American papaw (not to be confused with the 

 papaws of tropical climes, which belong to a 

 different family of plants) , which is not uncom- 

 mon in the Middle Atlantic States and is pre- 

 cariously hardy as far north as Massachusetts. 



The Ilex is the only known member of this 

 flora that might have flourished in an environ- 

 ment of scanty rainfall, for it appears to have 

 been an evergreen form or at least of a coria- 

 ceous, evaporation-resisting type. The differ- 

 ent species of Ilex, however, show a wide range 

 in their climatic requirements, and among the 

 large number of existing species (more than 

 200) some have succeeded in occupying all the 

 tropical and temperate regions of tiie globe 

 except western North America, Australia, and 

 New Zealand. In the United States the vari- 



