38 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4TH SER. 
California, and the peninsula of Lower California. Farther 
than this it is not now desired to follow them, though no doubt 
enough is now known of them to render it possible to recog- 
nize their equivalents in other parts of the United States. 
In the same paper Dr. Arnold has mentioned supposed oc- 
currences of Oligocene rocks at various points on the West- 
coast and has described a formation which he calls the San 
Lorenzo, which he doubtfully refers to this horizon. The 
fauna as there described is essentially Eocene, though it con- 
tains many species occurring in the lower Miocene as else- 
where known. It is quite likely, though not yet proved, that 
the Upper Eocene shales of the central Mount Diablo section 
should be correlated with the San Lorenzo. In the same way 
they may be correlated with the upper part of the Sespe for- 
mation described by Eldridge and Arnold’ as occurring in the 
mountains of Ventura county, and tentatively classed as 
Oligocene. 
The horizons of the Miocene can be safely correlated only 
within narrower limits, and it is not now intended to extend 
such correlation beyond the immediate environs of the Mount 
Diablo range. 
Homer Hamlin’* has described certain beds under the name 
“Vaquero Sandstone”, and Dr. Fairbanks* and Arnold* have 
repeatedly employed the same name in various papers. The 
type locality from which the name is derived, however, lacks 
thus far any faunal or even stratigraphical description, and 
as it can not be found on any published or official map of the 
state or county in which it is said to exist, it is difficult to 
decide what portion of the Miocene rocks, if indeed any, 
should be classed under this name. The locality has been 
loosely defined as the eastern slope of the Santa Lucia range, 
or the western side of the Salinas valley, etc. Hamlin’s de- 
scription is quite too meager to identify its position in the 
stratigraphic scale, and aside from suggesting that it is not 
1U. S. Geol. Surv. Water Sup. & Ir. no. 89, p. 14. 
2U. S. Geol. Surv. Bull. no. 309, pp. 10-12. 
3U. S. Geol. Surv. San Luis folio, p. 4 et seq. 
4Proc. Am. Phil. Soc. v. 43, pp. 19-20; U. S. Geol. Surv. Prof. paper 47, pp. 18-19; 
U. S. Geol. Surv. Bull. no. 309, pp. 12-17. 
