Cenozoic Floras Around the Northern Pacific 57 



Gravels of the Sierra foothills, but they are rare or absent at the 

 other localities, as are seeds and flowers. Leaves are the struc- 

 tures most commonly preserved, occurring in the tuffaceous 

 shales as impressions which show all the details of margin, 

 nervation, and texture. The propriety of referring these leaf 

 impressions to modern genera has been questioned on many 

 occasions by botanists who feel that generic characters are neither 

 constant nor recognizable in foliar structures. The paleobotanist, 

 however, usually dependent on the evidence of leaves, has been 

 forced to find in them criteria for recognition which are un- 

 known to most botanists; he sees in the arrangement of the 

 nerves, particularly near the margin, diagnostic characters by 

 which a relationship to modern genera can often be estabUshed; 

 with certain specimens he may be forced to indicate only a family 

 relationship by the use of such names as Laurophyllum, Legu- 

 minosites, or Menispermites. Familiarity with the foliage of 

 modern plants in forest and herbarium makes possible his refer- 

 ence of almost all leaf fossils to genera which indicate their sys- 

 tematic position, leaving only a small residue in the form genus 

 Phyllites. It is significant to note that botanists of wide experi- 

 ence in field and herbarium, such as Abrams, Jepson, and Mason 

 for temperate floras, and Gleason, Greenman, Merrill, Standley, 

 and Wilson for those of lower latitudes, are able to recognize 

 readily the leaves of modern woody plants, and to suggest the 

 generic or family affinities of fossil leaves as well. Acknowledg- 

 ment is due to these and many other botanists for the valuable 

 assistance which they have given to the writer and his associates. 

 As compared with the species of later Tertiary floras, Eocene 

 leaves are large and thick, with the margins more commonly 

 entire and with a consequendy greater development of campto- 

 drome nervation. The families which are most characteristic, 



