148 UNITED STATES GEOLOGICAL SURVEY— TERTIARY FLORA. 



range of 6° only, while Q. rubra, Q. alba, and Q. obtusiloba have their place 

 in the list of the species ranging from nineteen to twenty-nine degrees of 

 latitude. 



The disposition of Oaks to constitute varieties by hybridity is well 

 known. This has rendered the specification of many of their forms very 

 difficult and uncertain. It is therefore not surprising to find the same uncer- 

 tainty in regard to a number of the fossil forms referred to this genus, and 

 which are identifiable by leaves only, the most variable and diversified organ- 

 ism of all. 



Considering what is known of Oaks in geological times, species of 

 Quercus are positively and distinctly represented in the Cretaceous; at least, 

 if we admit as representatives of this genus the leaves of the Cretaceous of 

 Belgium, which have been separated by Debey and d'Ettingshausen, under 

 the generic name of Dryophylluin, for reasons which have not been as yet 

 satisfactorily explained. These leaves have the same characters as certain 

 species of Oaks ; and, compared for their nervation, their form, even their 

 consistence to those of some of the North American living species, they do not 

 present any mentionable difterence. The fine leaf, for example, described in 

 the Supplement to the Cretaceous Flora, Annual Report, 1874, as BryophyUum 

 {Quercus) lati folium, p. 340, pi. vi; fig. 1, is like a counterpart of some leaves of 

 Quercus bicolor; and that of Quercus primordialis of the Cretaceous Flora (Rep. 

 U. S. Geol. Surv. of theTerr., vol. vi, p. 64, pi. v, fig. 7) is equally similar to leaves 

 of Q. prinos. The section of the Willow-oaks, the Salicifolim, established by 

 Schimper for species with entire leaves, is less definitively recognized in the 

 Cretaceous. The characters of Quercus salicifolia and Q. cuneata, Newby., 

 like those of Q. EUsworthiana, Lesqx., do not exclusively pertain to Oaks; 

 therefore the relation of the leaves representing these forms is somewhat 

 doubtful. However, the evidence afforded to this question by geological 

 records seems to prove the origin of at least two different types of Oaks in 

 the Cretaceous ; for the Paleocene of France has five species, described by 

 Watelet, and, besides those which are mentioned here below, five others 

 have been recently observed by Saporta in the Gelinden formation, which, 

 considered for a long time as Upper Cretaceous, is now definitively recog- 

 nized as the lowest member of the Eocene of that country. Three of these 

 Oaks, according to the author's remarks, go to the section Lepidobalanus, 

 the two others represent that of Cerris. Higher into the Tertiary, the 



