fcfESy The Berkeley Hills. 393 



off These original surfaces, representing the more or less porous 

 scum or froth of the lava, have been thoroughly oxidized, and 

 present a reddish earthy or ocherous appearance quite different 

 from that of the fresh compact rock beneath. Unless their relation 

 to the main body of the lava sheet is understood, they are liable to 

 create confusion in the mapping, by giving rise to the suggestion 

 th.it the}- are distinct lavas. 



DESCENDING SECTION ON SAN PABLO SLOPE. 



Continuing our attempt at a systematic account of the sequence 

 of formations observable in a section of the Berkeley Hills in the 

 line of Skyline Ridge and its extension toward San Pablo Valley, 

 it remains now to describe the formations outcropping on the 

 northeastern side of San Pablo Ridge beneath the Siestan. The 

 Siestan, as has been already noted, being composed of soft beds, 

 occupies a gently-sloping shelf or terrace above the brink of a 

 steep escarpment overlooking San Pablo Valley. 



Basalts. — The basal beds of the Siestan are here limestones 

 resting directly upon hard basalts. These basalts have a thickness 

 of from 350 to 500 feet, and comprise several flows, which are diffi- 

 cult to segregate. These flows are separated by thin tuff bands or 

 by layers of brick-red lateritic accumulations. But, although these 

 separating lines can be made out fairly well on the shoulders of 

 the escarpment, they can not be correlated satisfactorily across the 

 ravines, so that the entire accumulation of lavas has been repre- 

 sented by one color on the geological map. 



Limestone Intercalated ivith Basalts. — hi the midst of these 

 basaltic lavas is a bed of whitish or light gray, compact, cherty 

 limestone, varying from 30 to 40 feet in thickness. The limestone 

 contains fresh-water fossils identical with those in the limestone 

 beds of the Siestan formation. These fossils have, in many cases, 

 been completely replaced by chalcedonic silica. This limestone 

 bed is well exposed on Eureka Peak and on the hill to the north of 

 it. The limestone bed dips conformably with the lava sheets to 

 the southwest, at angles varying from 30 to 6o°. The presence 

 of this thick bed of limestone, intercalated with the lava flows, is 

 significant of an important interval, characterized by lacustrine 



