124 University of California Publications in Geological Sciences 



dominants were restricted chiefly to moister bordering slopes and canyons, and 

 only the hardier stream-border types of the "West American Element were common 

 in the lowlands. The wide restriction of forest over lowlands of the Verdi area not 

 only demonstrates its Pliocene age, as Chaney has earlier noted (19386, p. 388), 

 but also suggests the flora is Middle Pliocene (Axelrod, 1948, p. 131). In this con- 

 nection, it is pertinent that whereas the large-leafed Populus eotremuloides 

 Knowlton is dominant in the nearby Chalk Hills flora, at Verdi it is represented 

 by the derivative, smaller -leafed P. alexanderi. This relation is also consistent 

 with a Middle Pliocene (Hemphillian) age. 



(2) In western Nevada, the Madro-Tertiary Geoflora was characterized by 

 woodland and chaparral, arid subtropical scrub being confined to warmer regions 

 at lower latitudes (Axelrod, 19506). There are no known records of this geoflora 

 in the nearby Middle Miocene Pyramid flora; it occurs in moderate numbers in 

 the Late Miocene Buffalo Canyon fllora ; it is abundant in the Mio-Pliocene Chloro- 

 pagus, Fallon, Aldrich Station, and Middlegate floras of west-central Nevada, and 

 dominates the Early Pliocene Esmeralda flora of south-central Nevada. To judge 

 from the small Truckee floras, it was represented chiefly by hardier stream-border 

 types and scattered oak savanna during the Middle Pliocene (Hemphillian), when 

 grasslands are inferred to have become more prominent at the expense of woodland. 

 The California Woodland and Chaparral Elements are no longer represented by 

 equivalent species in the region, though the Conifer-Woodland Element has per- 

 sisted down to the present day on slopes above the desert as pinon-juniper wood- 

 land. To judge from the nearby Chalk Hills flora, which is dominated by species 

 of the Arcto-Tertiary Geoflora, and in which Madro-Tertiary species are rare, 

 Madro-Tertiary species did not invade the east margin of the Sierran slope until 

 later in the Pliocene when the climate was drier. These relationships are fully con- 

 sistent with a Middle Pliocene age for the Verdi flora. A Late Pliocene (Blancan) 

 age seems unlikely for the climate was then cooler and moister, and the Madro- 

 Tertiary plants that now survive in milder regions probably would not have per- 

 sisted in the area to such a late date. 



One further point must be made with respect to plant evidence regarding the 

 age of the flora. It has been suggested that the percentage of genera in a fossil flora 

 that still live in the region of the fossil locality provides a measure of age because, 

 in general, the percentage decreases from 100 in Recent floras to a very low figure 

 for those of Early Tertiary age (Barghoorn, 1951). All of the Verdi genera still 

 live near the locality. Chara, Potamogeton, and Nymphaea are common in nearby 

 ponds and backswamps on the Truckee River floodplain, Ceanothus, Pinus, Popu- 

 lus, Prunus, Ribes, and Salix occur within 200 yards of the fossil locality, and 

 Abies, Arctostaphylos, and Quercus are in canyons on the north front of the Carson 

 Range 2 miles distant. Since all of the genera (100 per cent) are in the immediate 

 area, the Verdi would be of Recent age according to the method of age analysis 

 proposed by Barghoorn. The inherent weaknesses of the method as applied to age 

 analysis of Tertiary floras have been outlined elsewhere (Axelrod, 19576) and need 

 not be reviewed here. 



