62 
ASPHALTE GROUP—POLYTHALAMOUS BEDS. 
and determining its crested outline ; the eastern slope of the conglomerate rock is at an angle 
of 45°, serpentine lying underneath it and protruding on the western edge of the huttes. 
Further to the south these buttes converge in direction, or nearly so, with the upheaving rock of. 
the low hills along the coast, and both unite in closing the San Luis valley to the south, form¬ 
ing the elevated land of bTapoma, and thence south till the valley of Guadalupe Largo is 
reached. 
From these huttes the valley is a plain or a gradual slope from four to six miles, when the low 
hills between it and the coast are encountered ; towards these hills the waters roll and find 
their way partly out by low passes into the ocean, while in some places large ponds and 
marshes are formed by their collection. The low hills of the coast can he well seen immediately 
west of the town, whence the river, which waters the village San Luis Obispo, finds its 
way out west of the Corral de Piedras. The first stratum met with is a thick bed of con¬ 
glomerate and grit rock ; beds of quartzose pebbles, cemented by a calcareous clay paste, of a 
greenish yellow color, mostly made up of broken trap rock, a few serpentine and porphyry 
pebbles, are also enclosed, hut it is chiefly trappean, and is thus easily distinguished from the 
green conglomerates of the Santa Margarita valley. The thickness of this bed is not less than 
300 feet, including some intercalated layers of finer yellow grit with white quartz pebble ; 
above this is a yellow sandstone, soft, and easily disintegrated ; this sandstone is 150 feet in 
thickness ; then occurs the asphalt rock, a greenish yellow bed, where not highly charged 
with bitumen ; where it is, it is blackish, composed of fine grains of white quartz, cemented 
together by a calcareous and clay paste; s#me layers in this bed were highly charged with 
foraminifera; the total thickness of this asphalt group of rocks here might he about 120 feet. 
Upon this reposed a layer of soft, white felspathic clay rock, in a state of minute division ; in 
places so soft as to he readily cut with the knife ; in others, hard, and almost slaty ; in places 
partly calcareous ; in others, pure argillite, or kaolin clay. These beds were a little thicker than 
the foregoing, and perhaps were 200 feet in thickness. In these occurred the area obispoana, 
of Conrad’s report. As these beds were first encountered in this plain, and were afterwards 
found at several points along shore to the south as far as Los Angeles, and as they are almost 
always associated with asphaltum, they will deserve a somewhat fuller notice. 
The total thickness of these beds approaches 800 feet. From the Santa Lucia sandstones it 
is locally separated by the serpentine huttes and intervening valley. Its geological connexion 
could not be traced at this point, the lowland and swamp of the valley occupying a large 
surface. 
In the lower conglomerate no fossils were observed, nor in the yellow sandstone lying above 
it; the two upper beds contained fossiliferous layers: the asphalt rock containing the 
polythalamous shells, and the soft argillite containing impressions of area obispoana, this 
fossil cast alone being found ; while in other places, broken casts of fish scales and dorsal 
spines of minute dimensions were found scattered throughout. 
The upper thirty feet of this stratum is harder, and approaches a slate, readily splitting into 
thin laminae. This and the cream yellow tint, with the included fossil, serves to recognize 
the bed. 
The polythalamous layers were intercalated between strata of fine sand rock, made up of 
minute rounded grains of transparent quartz, not cemented, hut adhering by cohesion ; these 
could he separated at the edges by mere pressure of the fingers. Among the polythalamia, the 
forms of rotalina and orbicularia were the most abundant. 
