May, 1904. GEOLOGY OF WESTERN MEXICO—FARRINGTON. 207 
branching structure characterizes their arrangement. The strata 
exposed in the mountain peaks lie, for the most part, in a relatively 
horizontal position, although broad synclines and anticlines are, at 
times, well marked. A distinct bedding is observable in the structure 
of many of the peaks. These beds are often of great thickness (200 
to 500 feet), and usually lie conformably upon one another, although 
evidences of a quaquaversal structure are sometimes seen. Extensive 
faults were observed in one or two instances, but no opportunity was 
afforded to study them in detail. 
Of the exact nature of the rocks little can be said, as they were, 
for the most part, only seen from a distance. While the structure 
outlines of many of the peaks strongly suggest sedimentary rocks, 
eruptive rocks probably prevail. Ordonez, in his map of the rhyolites 
of Mexico,* represents this part of Mexico as characterized almost 
entirely by rhyolites. Upon what observations his conclusions are 
based, however, he does not state. | 
In going from the summit to the base.of the peaks and vice versa, 
all gradations of temperature were éxperienced, from cold temper- 
ate to tropical. At the Ciudad ranch a snowstorm raged, while 
at Ventanas orange-trees were in bloom. Another change indicating 
differences of temperature was one in the color of the soil, which 
on the higher levels was yellow, and on the lower red. The west- 
ern slope immediately along the edge of the plateau bore abundant 
vegetation, which at the lower levels was of a rank, semitropical 
character, with many brilliant flowering plants. Bird and insect 
life was extremely abundant as well. Passing westward, however, 
the character and amount of vegetation showed a rapid change to 
semi-arid conditions. Cacti and a few low shrubs succeeded the 
rank growths of the western slope and bird and insect hfe became 
scanty as well. These conditions prevail, the writer was informed, 
quite to the coast, so that it is evident that rainfall over the 
mountainous tract is slight. By means of irrigation, however, 
tropical fruits can be easily raised in the valleys, one “‘rancho’’ 
of this sort being conducted near Ventanas. Lemons grow wild 
about Ventanas, and many of the cacti bear fruits which are used 
for food. 
It is probable that the aridity of this area is caused by the 
warmth of the land being such as to prevent cooling of the winds 
from the Pacific to a sufficient degree to produce precipitation until 
they have reached the summit of the plateau. From this point to 
*Boletin d. Inst. Geol. de Mexico, No. 14. 
