746 N. H. DARTON 
The thin sheets of rhyolite included in this section thin out to 
the south, but thicken considerably to the north, and the mass of 
agglomerate also thickens greatly in that direction. This series 
appears again in ridges passing just east of La Paz and on the islands 
of Espiritu Santo, Partida, and San Josef, where it consists of thick 
bodies of agglomerate, tuff, and eruptive rocks. ‘The thinning and 
fining of the formation to the west is plainly visible in many 
canyons on the west slope of the sierra west and northwest of La 
Paz as far north as the Arroyo Santa Cruz near latitude 25° 20’. 
In this part of the peninsula the soft, gray, massive, typical ‘‘mesa 
sandstone” appears extensively in the highlands of the sierra as 
a component of the complex; but to the west, as the beds thin and 
fine, it becomes the dominant feature. It is exposed in the arroyos 
Conejo, Datilar, Guadalupe, 35-60 miles west of La Paz, and in 
the cliffs just below San Hilario; but at all of these places layers 
of pebbles are included and scattered bowlders occur. About El 
Pilon, to miles north of Hilario, it appears in many cliffs, some of 
which show limy layers. Farther north the conglomerate admix- 
ture increases notably in the vicinity of Rancho Jesus Maria, 35 
miles north-northwest; but at the north side of Cerro Nombre del 
Dios, 6 miles southwest of that ranch, the gray sandstone contains 
only a few thin beds of conglomerate and widely separated bowlders. 
The westernmost exposure observed in this portion of the peninsula 
was on the Arroyo San Luis, 17 miles northwest of San Luis, where 
the sandstone is fine-grained and massive. 
PLIOCENE TO POST-PLIOCENE 
Along the wide belt of lower lands adjoining the Pacific south 
of latitude 26° 30’, there is a cover of sand and gravel with lime- 
stone members which doubtless include not only the Quaternary, 
but probably also a formation of Pliocene age. These deposits 
vary in thickness from a few feet to 200 feet or more, and they 
extend far up the slopes, and on the higher mesas they are probably 
represented by a thick cap of bowlders. The latter are especially 
well exhibited on the high mesas north and northeast of San Hilario 
and on the long sloping mesas crossed by the main trail north from 
San Luis. The wide, low plains of the Magdalena region are 
EE ——— =< eel 
