DEPARTMENT OF BOTANICAL RESEARCH. 67 



occurring in the asphalt beds at the Rancho La Brea near Los Angeles, 

 in the deposits of Lake Lahontan, has raised the interesting possibility 

 of the continuance of this fauna well into the Recent period in Nevada. 



Progress in the Measurement of the Physical Factors of an Isolated Desert 

 Mountain, by Forrest Shreve. 



Instrumental observations have been continued at the seven stations 

 extending from 3,000 to 9,000 feet on the Santa Catalina Mountains, 

 from six of which data have now been secured for four years. The 

 readings include the summer rainfall and the absolute minimum tem- 

 perature at the seven stations, together with the absolute maximum 

 temperature at 8,000 feet. The soil-moisture on both north and south 

 slopes at the seven stations is determined for the period of its annual 

 minimum in the arid fore-summer, and the absolute maxima and 

 minima of soil temperature have also been secured during the past 

 year. The soil-moisture on north slopes at 9,000 feet is approximately 

 as great at the time of its annual minimum as it is on the desert at the 

 time of its annual maximum. The absolute minimum temperatures 

 at the highest stations are found to be much higher than the absolute 

 minima for the same winters at stations of the Weather Bureau which 

 are located at the same elevations on the Mogollon Plateau in central 

 Arizona, indicating that the isolated desert ranges present less severe 

 winter conditions than do the extended plateaus of the same elevation 

 in nearly the same latitude. The minimum soil temperatures at all 

 of the stations are found to range much higher than the atmospheric 

 minima at the same stations, a condition of great importance in the 

 growth activities of plants. 



The record of rainfall at 8,000 feet now extends through eight 

 summers and affords a striking contrast to the summer rainfall at the 

 Desert Laboratory, not only in its amount, but in the lack of relation 

 between the annual fluctuations in the two localities, which are sepa- 

 rated by a vertical distance of 5,300 feet and a horizontal distance of 

 20 miles. The evidence is becoming more conclusive from year to year 

 that the rainfall at 9,000 feet will prove to be less, on a long average, 

 than that at 8,000 feet. (Fig. L) 



Comparison of the Vegetational Gradients of Isolated Desert Mountains, 



by Forrest Shreve. 



The work on the correlation of vegetation and climate in the Santa 

 Catalina Mountains, which has been under preparation for publication 

 during the year, has been extended by an examination of several other 

 isolated desert mountain ranges in southern Arizona. The object has 

 been to discover the possible influences of the character of the under- 

 lying rock and its soil in causing departures from the distributional 



