732 N. H. DARTON 
The principal rocks of the Monterey beds are gray to pale- 
buff, fine-grained sandstone and sandy shale with abundant fish 
scales. They include interbedded diatomaceous deposits mostly 
from 1 to 3 feet thick and some thin layers of glassy silica from 
buff to black in color. More than 500 feet of beds are exposed 
along the Arroyo de la Purisima where for several miles the forma- 
tion extends from 5 to 20 feet above the bed of the arroyo and 
there are many flexures. 
The southernmost exposures in the Arroyo de la Purisima in 
the west end of the Big Bend about 2 miles above tidewater contain 
much fine, white diatomaceous earth interbedded in fine-grained, 
yellowish sandstone and compact shale with many fish scales. In 
Fic. 9.—Flexures in the Monterey beds on the Arroyo de la Purisima, ten miles 
below La Purisima, Baja California. 
this region the dips are to the west or southwest, so that higher 
beds rise as the valley is ascended. In about 3 miles the lowermost 
beds appear in the crest of a low anticline, and from these were 
obtained the following fossils, determined by Dr. Julia A. Gardner: 
Scutella andersoni; Chrysodomus sp.; Turritella sp., cil. T. mar- 
garitana Normand; Vermetus sp.; Leda sp.; Arca, cf. A. medio- 
impressa Clark; Pecten, cf. P. crassicardo Conrad; Crassitellites 
(Crassinella) sp.; Cardaum sp. nov.; Chione sp.; Cytherea sp.; 
Mecoma sp.; Solen sp.; and some corals and bryozoa. The age is 
regarded as probably Vaqueros, equivalent to Lower Monterey. 
These lowest beds appear again in another small uplift a short 
distance north, beyond which the strata descend in apparent regular 
order showing extensive exposures in the banks of the arroyo as 
shown in Figure9. Here the formation is overlain by conglomerate. 
