178 Tertiary. 



Ill 1856, W. P. Blake* described the Tertiary rocks of the vicinitj' of 

 San Francisco, California. They consist of fine-grained, compact sand- 

 stone, associated with shales, and underlie the city of San Francisco, 

 and are exposed along- the shores of the bay, both north and south of 

 the city, forming the principal promontories and points, and several 

 islands. On entering the bay from the Pacific, the}^ are first seen at 

 Point Lobos, the outer point, and again at North and Tonquin points. 

 They border part of the Golden Gate on the north, and form the shores 

 of Richardson and Sancelito ba3^s. Angel, Yerba Buena and Alcatrazes 

 Islands, are of the same age. In some places, hills and ridges of 200 

 or 300 feet in elevation are formed entirely of this sandstone. Rocks 

 of the same age are found at Benicia, New Almaden, and between San 

 Juan and Monterey. 



On the south end of the Island of Yerba Buena, a section, 200 feet 

 thick, shows the sandstone layers, varying from a few inches to six 

 and eight feet, and alternating with beds of argillaceous slates and 

 shales. All the weathered surfaces of this series of beds are of a rust}^- 

 brown or drab color, which extends throughout the rock to a depth of 

 from ten to twenty feet, down to the limit of atmospheric influences. 

 There are parts, however, of the upper beds that have not been reached 

 and changed by decomposition; these parts are found in the condition of 

 spherical or ellipsoidal masses, from which the weathered parts scale off 

 in successive crusts. These nuclei have the appeai'ance of great, rounded 

 bowlders, and have accumulated, in great numbers, at the base of the 

 cliflL". They are of various sizes, but are smallest in the upper parts of 

 the strata, near to the surface. 



This spherical or globular condition does not appear to be the result 

 of any peculiar arrangement of the material of the strata, a concretion- 

 ary action, such as takes place in the igneous rocks, but is probably 

 due to decomposition, the result of the absorption of infiltrating waters 

 charged with imparities. A solid and homogeneous cube of sandstone 

 thus exposed, under conditions favorable for absorption of the water on 

 all its sides, would decompose most rapidly on the angles, producing a 

 succession of curved surfaces gradually approaching a sphere. 



The color of the sandstone is dark, bluish green, inclining to gray. 

 It is exceedingly compact and tough, and does not break so readily as 

 the fine-grained, red sandstone of the Connecticut river and New Jersey 

 quarries. 



* Explorlitions and Surveys for a railroad from the Mississippi river to the Pacific ocean, 

 vol., V. 



