Mesozoic and Caenozoic Geology and Paloiontology. 1S7 



prett}- uniform thickness ; of tliese, bed 4, an arenaceous, yellow clay, 

 is described as containing small marine shells. The brownish loamy 

 cl;iy (bed 1), is exposed by ever}' creek, and in the sections produced 

 by the Los Angeles river, several feet of the bluish clay (2) are ex- 

 posed ; the beds are deposited almost perfectly horizontal, and are, 

 therefore, unconformable to the soft sandstones of the San Pedro 

 hills and the Sierra Monica, which in the former case have a dip of 20 

 deg., and in the latter are in places almost vertical ; thej^ have, there- 

 fore, been deposited posterior to the upheaval of these soft Tertiary 

 sandstones, and the surfaces have undergone no material alteration of 

 contour since, the only change being that of elevation of the whole 

 region out of the bed of the sea. 



An investigation into the mineral nature of these various deposits 

 shows that the alluvial covering, to the depth of six or seven feet, is 

 aluminous in its finer parts, and granitic pebble in its coarser, and has 

 been the result of the degradation of granitic and felspathic rocks. 

 The soil of the plain is rarely quartzose, except when close to some of 

 the low Tertiary hills, which alteration may therefore be due to the 

 wash of these latter. 



Tlie blue clay is generally assigned by geologists to a slow deposit 

 of mud, produced hy the sifting action of the tide in estuaries or gulfs 

 where matter is not transported by current actions ; it is the evidence 

 of a calm condition of the waters during the period of deposit, and a 

 cessation of upheavals of the land contiguous ; the two beds of bluish 

 clay are separated by nearly forty feet of gravel and sand. 



The drift gravel (bed 3) consists not only of rounded granitic 

 pebbles, but also those of syenite, hornblende schists, metamorphic, 

 brown sandstones, trap and amj-gdaloid ; and the underlying sandy 

 bed is chiefly quartzose, and probabl}' is the detritus of the sandstones 

 at the base of the Cordilleras. 



There have been no very large stones seen in the drift beds, there 

 are no loose bowlders or erratic blocks, nor is there, either on the sur- 

 face or in the deposits, any stone which can not be traced to masses of 

 similar mineral constitution in the ranges bordering the plain. The 

 period of general or polar drift, therefore, which was one of the earliest 

 of the Post-pliocene epoch, passed by without affecting California ; 

 and it was during the later periods of drift that the processes of wear- 

 ing down continents and depositing them in the seas around took 

 place, and were carried out on an immense scale, and over an immensely 

 extended period of time. 



Los Angelos plain is not the only one in California, where these de- 



