292 SHREVE: ACROSS THE SONORAN DESERT 
Idria are globose, and all of the thickening of the stem is above 
ground. 
The heaviest stands of Jdria were along the base of the gran- 
itic hills. It is infrequent on the upper slopes, and nowhere 
grows very far away from rock in place. Unlike most plants 
at the northernmost limit of their occurrence, Idria is more 
abundant on north slopes than on south slopes. It was found 
inland from Punto Kino as far as the hills extend, and southward 
as far as the second ridge of hills below the point, which marked 
the southern limit of our exploration. 
Throughout these hills [dria is the most conspicuous plant, 
Pachycereus, Lophocereus and Lemaireocereus are all infrequent, 
although they may be found in the valleys and on alluvial slopes. 
Carnegiea is confined to the hills, but nowhere reaches its custo- 
mary size. Parkinsonia is very uncommon, and Covillea is 
confined to coarse detrital slopes at the bases of the hills. On 
the slopes there are few large individuals of any plants. The 
most characteristic forms are Elaphrium Macdougalii, Olneya 
tesota, Idria columnaris, Jatropha cinerea, Simmondsia cali- 
formica, Hyptis Emoryi, Opuntia Bigelovii, Fouquiera splendens, 
Encelia farinosa, Franseria dumosa, Trixis angustifolia, Ver- 
besina chihuahuensis, Porophyllum gracile, Hibiscus denudatus, 
Stegnosperma halimifolium and Ptiloria pauciflora. 
The vicinity of Libertad and the hills of Punto Kino were of 
particular interest as examples of a pronounced desert running 
right down to the shores of a large body of water. While the 
temperature conditions here are extremely favorable there is 
every evidence that the precipitation is much less along the coast © 
than it is 10 miles and more inland. In November, 1923, the 
writer installed non-evaporating volumetric rain gauges at 
Libertad and at the pass in the Sierra Picu 18 miles from Liber- 
tad. At the latter locality a Six’s maximum-minimum ther- 
mometer was also installed. These instruments were read in 
March, 1924, by Mr. Gilbert Sykes. The precipitation at 
Libertad was 1.0 cm. and at Picu pass 4-8 cm., while for the 
same period at Tucson it was 11.2 cm. The minimum temper- 
ature at Picu Pass was 34° F., and for the same period at Tucson 
27.5°F. These figures indicate the extreme aridity of the coastal 
belt and show that the precipitation of the hills nearest the coast 
is considerably higher. The minimum temperature reading 
