G2 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 buttes. 



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 Napoma, and thence south till the valley of Guadalupe Largo is 

 reached. 



From these buttes 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 be 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, but 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 ; some layers in this bed were highly charged with 

 foraminifera ; the total thickness of this asphalt group of rocks here might be 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 be 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 buttes 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 

 poly thalamous 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 lamina}. 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, but adhering by cohesion ; these 

 could be separated at the edges by mere pressure of the fingers. Among the polythalamia, the 

 forms of rotalina and orbicularia were the most abundant. 



