202 DESCRIPTION OF THE PLATES. 



Fig. 2. — Section from the slope of the incoherent sandstones to Soda lake, the debouche of Mo- 

 jave river. 

 f, loose sandstone ; a, bed of Mojave, with alluvium ; h, conglomerate lying up a hill of 

 granite ; gr, cut through by felspathic porphyry ; o, deep bed of local drift covering up 

 the sandstones, and forming the surface of the plains and valleys east of the Cordilleras ; 

 c, second crossing of the Mojave, near Navajo camp ; /', loose sandstones uplifted by the 

 porphyry dykes, (red amygdaloidal trachyte ;) g, albitic granite of the ranges running 

 north and south ; I, raetamorphic limestone lying near granite of Soda lake. 



Fig. 3. — This section shows the great slope of San Bernardino plain, c, with its deep alluvial 

 beds, and the upper bed of drift conglomerate, h, removed ; g, the granitic mass of Kikal 

 Mungo mountain ; s, the syenite of its eastern edge ; a, mica slate and gneiss rock ; 

 /, the soft sandstones dipping from the Cajon to the Mojave river. 



PLATE VII. 



Fig. 1. — Section from the mouth of the Santa Clara river to the mouth of the Gila, at Fort 

 Yuma. This section displays the promontory form of southern California, and the low 

 level of the Colorado desert. 

 a represents the alluvium of the plain on the Pacific slope of the Cordilleras, and a' the 

 alluvium of the Colorado basin ; &, the sandstones of the Cordilleras ; c, the upper sand- 

 stones of the Monica group ; gr, the granitic rock of Kikal Mungo and of Fort Yuma ; 

 t, trachyte of the Monica range ; v, unconformable tertiary beds at Carrizo creek. 



Fig. 2. — Section from the valley of Santa Margarita t3 the ocean, through the pass of San Luis 

 Obispo, and across San Luis valley to the sea. 

 a, a, a, alluvium ; 6, 6, i, brown and yellow sandstones of the Santa Lucia mountains ; c, coarse 

 conglomerate ; s, s, serpentine, the axial rock ; /, /, amygdaloidal felspathic trap ; t, t, t, 

 augitic traj), in veins and dykes. 

 d. Conglomerate underlying the San Luis asphaltic group; e, soft yellow sandstone; g, 

 greenish yellow bituminous rock ; h, soft white argillite rock, containing casts of Area. 

 The layer e often includes layers containing polythalamous shells. 



PLATE VIII. 



Geological structure from San Diego to Big Horn mountains, east of the Colorado river. 



Figure 1. Section from the Pacific ocean across the Sierra Nevada via Warner's pass, show- 

 ing the trappean rock at San Pasqual, the granitic mass of the Cordilleras, and the 

 tertiaries of tlie desert at Carrizo. 



Figure 2. Profile from Sackett's wells to Big Horn mountains : a. Alluvium, fine sand and 

 clay ; G. Hornblendic granite ; S. Sandstone rock, a coarse conglomerate, with pebbles 

 of volcanic rock ; T. Trachyte and amygdaloid porphyry. 



Figure 3. Enlargement of part of section 2 to show the structure of the Dome hills and Big 

 Horn mountains: a. Alluvium, fine gravel and sand ; A P. Trachytic porphyry, a fels- 

 pathic rock ; T. Metamorphic quartz rock, witli jasper ; C. Conglomerate, as in section 

 2 — the desert conglomerate. 



Figure 4. Section of an anticlinal axis near camp at Vallecitas : G. Granitic axial rock ; M. 

 Gneiss and mica slate dipping away east and west, strike of these rocks N. 75° W. 



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