SHREVE: ACROSS THE SONORAN DESERT 293 
shows that there are at least occasional winters without frost, 
a thing that can not be said of the more elevated portions of the 
Sonoran Desert. At Libertad and Punto Kino freezing tem- 
peratures doubtless occur only at rare intervals and for periods 
of very brief duration. 
DESERT LABORATORY 
UCSON, ARIZONA. 
