1 ,AWSON ~| 

 HALACHEJ 



The Berkeley Hills. 



437 



volcanic series, in the presence of both quartz and hornblende. The 

 latter, which is uralitic, is probably secondary, and may be derived 

 from augite, but as this process has not been observed in any of 

 the earlier volcanic rocks, even in fragments included in this same 

 tuff, its presence appears to indicate a different origin for the 

 containing rocks. 



A specimen of the finer-grained portion of the tuff was found to 

 contain 68.8 per cent of silica. This indicates that the compo- 

 sition of the tuffs as a whole agrees with that of the acid inclusions 

 (microgranite) described above. 



The Basic Tuffs. — Owing to their decomposed and altered char- 

 acter, but little results from a microscopic examination of the more 

 basic and dark-colored tuffs. In general it may be said that the 

 volcanic fragments, in so far as they have been examined, appear 

 to be andesitic, some glassy, and others holocrystalline. In the 

 fresher specimens, such as are naturally selected for examination, 

 such andesitic fragments, together with crystals and crystal frag- 

 ments of plagioclase and augite, are imbedded in a very fine- 

 grained matrix, which is commonly so obscured by the products of 

 oxidation that little can be made of it. When this oxidation is 

 advanced, the tuff may be of a red color, and, in the finer-grained 

 varieties, it is then difficult to distinguish it from the lateritic 

 surfaces of the lavas, or the ancient muds arising from the wash 

 of such laterite. 



THE SEQUENCE OF LAVAS. 



The volcanic rocks of the Upper and Lower Berkeleyan and 

 Campan series comprise both lava flows and pyroclastic formations. 

 Of these pyroclastic formations the rhyolitic tuffs and agglomer- 

 ates stand out prominently in contrast to the more basic rocks with 

 which they are' intercalated. There are, moreover, no rhyolite 

 lavas in our field with which they may be correlated. The more 

 basic tuffs, on the other hand, seem to be referable to the andesitic 

 and basaltic lavas, with which they are associated, except for 

 certain thin tuff beds low down in the Orindan formation on San 

 Pablo Creek, and similar tuffs in the Siestan formation, for which 

 we have no known corresponding lavas. If we omit these basic 



