338 University of California Publications in Geology [Vol. 12 



two sections are similar in containing shales of the same general 

 texture, similarly interstratified with bands of sandstone and nodular 

 sandy clays; but the great outcroppings of mixed clayey sandstones 

 are only visible in the ledges. Professor Louderback, who has very 

 kindly made an analysis of some of this material, has reported as 

 follows : 



The rock appears to carry no tuffaceous material, but is probably derived 

 entirely from the erosion of the granitic terrane wih a subordinate amount of 

 metamorphic rocks and some veining material. The fragments are not well 

 rounded; some of the smaller ones are very irregular. All of the material except- 

 ing the very finest parts of the matrix is remarkably fresh and free from even 

 partial weathering. The finest material is in part altered to clay, and there are 

 some streaks of limonite here and there in the rock. The rock would correspond 

 to unsorted finer grades of alluvial material developed under arid conditions, or 

 at least under conditions where disintegration was in advance of weathering. 



These hard ledges, especially their lighter colored, finer, and more 

 calcareous layers, are at spots remarkably rich in fossilized teeth, 

 bones, small shells, and particles of wood. The first fossils were 

 found by the writer, minute bone fragments lying in open spots 

 along the brush-grown top. Other material was later detected by his 

 assistant in the weathered surface of the cliffs in process of decompo- 

 sition and permeated with root hairs. There the main work was 

 carried on by systematic excavation, great blocks of the rock being 

 detached, carefully examined and broken open. Much good material 

 was necessarily lost. Mining on a small scale was finally attempted 

 by the aid of dynamite. The small amount of determinable material 

 in the enormous quantity of the indurated matrix made the work 

 very difficult. 



PoTRERO Creek Deposits 



It was mentioned above that southeastward the Eden formation 

 rested upon arkosics of Lamb dome and the San Jacinto foothills. 

 The arkosics of the Lamb region first appear at the mouth of Lime 

 Kiln Canon (lying to the immediate west of Lamb Caiion) in the form 

 of a massive, south-dipping deposit of coarse, yellowish clay inter- 

 streaked with reddish arkose. The general eastern strike carries the 

 section across Lamb Caiion, where it abuts against the granites and 

 metamorphosed limestones of the Lamb Dome as well as against an 

 intervening deposit of redder arkose, and thence to the adjoining 

 valley of the Potrero lying at the foot of the outlying San Jacinto 



