GEOLOGIC RECONNAISSANCE IN BAJA CALIFORNIA 743 
cut the mesa zone, notably in the arroyos San Raimondi (or Miguel), 
San Gregerio, De la Purisima, Comondu, San Benancio, San 
Andres, and Santo Domingo. The greatest thickness observed 
in this area is 30c feet; to the west the beds are thin out to 
nothing under the lava caps of the mesas, and to the east they 
merge into the great formation of detrital and volcanic rocks 
constituting the central sierra. ‘This transition is well exposed in 
the Arroyo San Raimondi near the little pueblo of San Miguel, 
in the canyon of the Arroyo San Gregorio-above Purisima Vieja, 
in the Arroyo de la Purisima above Huerta Vieja (a few miles 
above La Purisima), and it is also evident in the canyons from the 
Arroyo Comondu southward. The change to the east consists 
in the sediments becoming coarser and thicker, and probably over- 
lying beds also are present under the great sheets of the basalt 
which cover the high mesas. Close scrutiny would probably 
reveal the presence of several formations in the sandstone series. 
A feature of this kind is clearly apparent about La Purisima where 
a harder, massive sandstone appears as a huge wedge in the other 
sediments (see Fig. 17). It thins out to the southwest, a mile 
or two below La Purisima. 
The eastern extension of the mesa sandstone was not studied 
in detail, but was observed in various trips across the high sierra. 
The predominating rock is conglomerate in thick beds and consist- 
ing largely of bowlders of igneous rocks together with some quart- 
zite. Sheets of lava, layers of tuff, and thick bodies of agglomerate 
are included. Some of the lavas are basalt, others are light- 
colored, more acidic rocks. To the east many intrusive masses 
are present. These are conspicuous west of Santa Rosalia, near 
Mulegé, east of Conception Bay, and southward past Loreto. 
A fine exposute of mesa sandstone on the west bank of the 
Arroyo San Gregorio, 10 miles slightly north by west of La Purisima, 
shows the following strata: 
ILANvay SINGEE BVETMOID OP WANES, boos sasdaancedo og sou oo-++ 
Sandstone, gray, massive, compact.:-..-.........- 60 - 
Samed stoneywiiteSOltmc yt bite -ko eater eer 120 
Bowldersvand: Sandi: 36 7/5. Sic ea etre sere sleepers ater 40 
Agglomerate, reddish in places..................-. 4 
Yellow beds in bottom of the arroyo............. 30 
