116 University of California Publications in Geological Sciences 



nearly resembles the fossil occurs generally at altitudes fully 500 to 1,000 feet 

 lower, where winters are more mild. And it is only at these lower levels that oaks 

 and other mild-winter types which have equivalents in the flora are to be found. 

 Although climatic data for intermediate elevations are not available for southern 

 California, they can be estimated in a general way by comparing the data for Mill 

 Creek (table 5) with those at Mount Wilson and Idyllwild. The Mill Creek station 

 is at an elevation of approximately 3,000 feet in the San Bernardino Mountains. 

 The area is dominated by chaparral and sage, with woodland in the region, and 

 with the lower margin of yellow pine forest 3 miles distant and 1,000 feet higher 

 (pi. 16, fig. 2). As compared with Mount "Wilson and Idyllwild, intermediate tem- 

 peratures may be inferred for these slopes where tongues of forest are in close 

 proximity to the small-leafed ecotype of P. trichocarpa. 



On the basis of the preceding discussion, the following temperatures are inferred 

 for the Verdi locality at the time of plant deposition: 



Winter temperatures 



Average January temperature... 45° F. 



Average January minimum 35° 



Extreme winter low 10° 



Summer temperatures 



Average July temperature 73° F. 



Average July maximum 85° 



Extreme summer high 100° 



Slightly lower temperatures prevailed in the bordering hills where the Sierra- 

 Cascade Component lived. In the basin proper the growing season probably was 

 not less than 200, and may have been as much as 230 days, as compared with 145 

 today. 



Populus subwashoensis is the only Verdi species that does not have a close rela- 

 tive living in California. It most nearly resembles the smaller leaves produced by 

 P. tremula davidiana of northeastern Asia. The Verdi poplar appears to be a 

 small-leafed derivative of the Miocene P. washoensis Brown, which has leaves that 

 are similar to those of P. tremula and P. grandidentata of the eastern United 

 States. Similar relationships between Middle and Late Teritary species have al- 

 ready been reported for such plants as Populus voyana-P. pliotremuloides and 

 P. eotremuloides-P. alexanderi, as well as for species of Acer, Amelanchier, Rosa, 

 and other angiosperms. The smaller-leafed Late Tertiary species appear to repre- 

 sent plants that were adapted to the drier, sunnier climate of post-Miocene time 

 (Chaney, 1938a, pp. 212-213; 1944, pp. 16-17; Axelrod, 1941; 1944a"). 



The climatic significance of P. subwashoensis lies in the fact that its nearest 

 modern relative survives in a region of summer rain. Many fossil plants whose 

 descendants are restricted now to temperate regions with ample summer rain, such 

 as beech, chestnut, elm, hickory, hornbeam, tupelo, and swamp cypress, were 

 common as far south as central Nevada in the Miocene. They gradually disappeared 

 from the far West as summer showers were reduced in amount and in frequence 

 during the Pliocene. The presence of only one species at Verdi whose nearest 

 relative is found in a summer-wet area indicates that precipitation was greatly 

 reduced in the warm season as compared with earlier in the epoch. 



