SOUTHEEN CALIFORNIA. 



85 



The summit and sides of these knolls are strewn over with the erratic fragments which pertain 

 to the continuous desert formation, and which conceal the central nucleus, except in the deep 

 cleft made by the passage of the Colorado just helow its junction with the Gila. At this point 

 we see the central nucleus forming an irregular breccia, composed of variously sized blocks, 

 frequently massive. The rock is a form of greenstone, streaked with epidote. The cementing 

 material is apparently derived from the natural decomposition of the contained rock. On the 

 western side, near the level of low water, there is an underneath exposure of a dark-colored 

 mass, composed of epidote. with talc — the latter being occasionally granular. This material is 

 seen shooting upward into the superincumbent breccia in the form of veins, and would seem to 

 hold some close relation with the disturbing cause below. 



The external features of this singular formation we have endeavored to represent in sectional 

 sketches. — (See part 1, pages 128, 129.) 



The Tertiary formation. — The Tertiary formation which flanks the mountain range on the 

 west extends to the Pacific coast, forming a belt of variable extent and thickness, and composed 

 of various stratified deposits. 



The strata adjoining the sea occasionally presents abrupt ocean bluifs, washed by the waves 

 at high tide ; through the same formation the various streams cut their way, forming deeply 

 trenched valleys. The strata thus exposed in the sides of valleys exhibit a slight dip toward 

 the sea, and are seen to be composed of alternate beds of coarse sand, clay, or marl, with occa- 

 sional beds of interstratified pebbles, all smoothly rounded, and variable in size. 



SECTION" OF LIGNITE BLCFF KEAK SAX DIEGO. 



y J-" — '\ '•■ e / ' -^ \ » -r J- ; 



py 



■VS1IM»" ^^^ ' • tt'tilflHIl't 



'mm ' 



Scach 



A. Coarse, feruginous sand, with occasional interstratified pebbles, weathering very irregularly into fantastic shapes, 

 forming miniature peaks and ridges. 



B. Coarse, white sand, in even, horizontal strata. 



C. Drab-colored sandstone. 



D. Fine-grained sand, varying in color from buff to light-gray, containing thin seams of sulphuret of iron. 



E. Tough clay, containing an irregular seam of lignite, with smaller portions of mineralized bitumen. 



In the vicinity of San Diego, and plainly exposed in the steep bluffs which bound the lower 

 part of the river valley at this place, is observed a distinct fossiliferous layer, having a thickness 

 of about thirty feet. Its lower members, resting on micaceous sand or clay, are not more than 

 twenty feet above the present sea level. 



The contained fossils are made up mainly of silicified casts of marine shells, imbedded in a 

 calcareous medium of more or less close texture. The individual forms of fossil species are 

 quite numerous, though the number of distinct species is not great ; the most abundant and 

 widely spread is a species of TurriteUa. — (See Plate No. XIX, fig. 8.) 



