742 N. H. DARTON 
formations constituting mesas in the central and northern parts of 
the peninsula, the name may be useful in the southern part of the 
region until a more definite classification is practicable. I find that 
the formation presents two phases: a massive, gray sandstone, 
several hundred feet thick in the western portion of the mesa 
region, rapidly merging into conglomerates with thick bodies of 
agglomerate and tuff to the east. This relation is shown in sections 
11-20, Figures 3 and 4, and is a most striking feature. The coarse 
sediments are in hard, massive beds, 4,000 feet thick in places, 
constituting the high sierra extending continuously southward 
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i 
Fic. 22.—Mesa sandstone on the Arroyo San Andres, twenty-one miles south of 
Comondu, Baja California. Shows massive bedding. 
from near latitude 28° to beyond La Paz. Sheets of contemporane- 
ous igneous rocks are included and the succession is penetrated by 
many intrusions, notably in the Mulegé and Santa Rosalia regions. 
To the east it lies on schist, granite, etc., and to the west on sand- 
stone of earlier Tertiary age. South of the latitude of La Paz the 
thickness diminishes, volcanic rocks are not present, and at Todos 
Santos the underlying rocks reach the shore of the Pacific Ocean. 
I did not observe its relations north of latitude 28°. 
In the area of the great, lava-covered mesas extending along 
the Pacific slope past San Ignacio, La Purisima, and Comondu, 
the yellow beds are overlain by massive, gray, mesa sandstone. 
This sandstone is conspicuous in the walls of many canyons which 
