^alachb] The Berkeley Hills. 407 



It is practically impossible to say how many flows are repre- 

 sented in the basalt formation, but, judging from the variation of 

 petrographic character and the presence of certain brick red depos- 

 its, probably lateritic, there must be several flows. These have a 

 maximum aggregate thickness of about 350 feet towards the south 

 and thin out to the north, so that beneath the rhyolite agglomerate 

 of Rough Hill the thickness does not exceed fifty feet. 



The latest formation is a light-colored, strongly-cemented 

 agglomerate and tuff, of which only residuary patches remain to us. 

 From the nature of this formation it is certain that it must have at 

 one time extended in a continuous sheet over a large part of the 

 area of the Berkeley Hills. At present it is represented by ten 

 small isolated areas, of which six do not exceed an acre in extent. 

 The largest of these remnants and the most instructive for study is 

 that which occupies the greater part of Rough Hill. Here it is 

 about iod feet thick, and shows only occasionally evidences of 

 stratification. It is a typical coarse fragmental rock of volcanic 

 origin. It presents bold and rocky outcrops, broken, by surface 

 disintegration, into large slabs and rounded, boulder-like masses, 

 the surfaces of which are studded with angular knobs and irregular 

 protuberances caused by differential weathering. On fresh surfaces 

 the rock is yellowish white, gray, or green, varying according to 

 the degree of oxidation. A light-colored, fine-grained paste 

 incloses fragments of quartz and feldspar and sharply-angular 

 fragments of various rocks of all sizes up to six inches in diameter. 

 This ground-mass may be seen with a lens to be heterogeneous in 

 composition and to be made up of finely-comminuted and sharply- 

 angular fragments of rocks and crystals. The abundance and 

 large size of the fragments in certain beds show that the vent from 

 which they were ejected could not have been far distant. The 

 location of this vent is, however, at present unknown. 



The other smaller patches represent only the basal portion of 

 the formation, and are in general not so coarse-textured, the angular 

 blocks of lava being usually not greater than an inch or two in 

 diameter. There are, however, no other important differences. 

 These patches do not all, however, repose upon the basalt. Two of 

 them lie across the line of demarkation between the basalt and 



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