xvii, 106 p. : ill., maps. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / This dissertation establishes the age, depositional environment, composition, and climatic conditions for the Cape Blanco flora. The paleotemperature estimated by the Cape Blanco flora, the Temblor flora of California, and the Seldovia flora of Alaska are then compared with sea surface temperatures estimated from oxygen isotope analysis of benthic foraminifera. The unconformity-bound shallow marine sandstone of Floras Lake includes a redeposited tuff bed which contains fossil leaves at Cape Blanco. An 40 Ar/ 39 Ar age of 18.26 ± 0.86 Ma is presented for the tuff as well as a paleomagnetic stratigraphy of the sandstone. Sedimentary structures of the tuff bed are evidence that the tuff was deposited at or just above the strand line. The depth of tuff deposition was shallower than the adjacent marine sands, and this short-lived shoaling may have been a result of increased sediment supply. The fossil flora was an oak forest with numerous species of Fagaceae. Additional components include lanceolate Salicaceae leaves, entire margined Lauraceae, fragmentary Betulaceae, and lobed Platanaceae. Coniferous debris, charcoal, Equisetales, and Typhaceae forms are also figured. Ten leaf forms could not be confidently assigned to established names but are described, figured, and called angiosperm forms 1-10. In total 44 unique forms are identified. The size and margin type of the dicot specimens are quantified, and by comparison with known modern floras, a former mean annual precipitation of 201 (+86, -61) cm and a former mean annual temperature of 18.26 ± 2.6°C are estimated. The paleotemperature of the ∼17.5 Ma Seldovia Flora and the ∼17.5 Ma Temblor Flora are estimated using the same method, establishing a ∼0.7°C per degree of latitude temperature gradient for the northern hemisphere temperate zone. The leaf based gradient is steeper than the sea surface temperature gradient, of ∼0.26°C per degree of latitude as estimated from oxygen isotopic composition of foraminifera collected from ocean sediment cores. Both fossil leaf and isotope methods suggest that the early Miocene was ∼5°C warmer than today. This thesis includes unpublished co-authored material. / Committee in charge: Gregory Retallack, Chairperson, Geological Sciences; Rebecca Dorsey, Member, Geological Sciences; Joshua Roering, Member, Geological Sciences; Barbara Roy, Outside Member, Biology