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University of California. 



[Vol. 2. 



augite, are imbedded in a very fine-grained paste, which is partly finer 

 ash and partly a brick mudstone referable to the wash of the 

 laterite of the underlying andesite. Part of the material mapped 

 with this deposit may, indeed, be the sedentary laterite of the ande- 

 site, for it is difficult to draw the line between the two things. 

 Some small patches of vesicular lava occur in it, representing 

 unimportant flows. 



Above this basic tuff and gravel layer, occurs a thin sheet of 

 dark', coarse, doleritic basalt, which is not indicated by a separate 

 convention on the map. It nowhere exceeds 20 feet in thickness. 



Rhyolite Tuff. — Lying on this is the third member of the forma- 

 tion, the most interesting, most voluminous, and most persistent 

 of the three. It is a thick bed of stratified rhyolite tuff. It appears 

 as a fine to medium grained rock, of compact to porus texture, and 

 a color varying from dirty white to tawny yellow. Its aspect is 

 distinctly that of a clastic rock. Grains of quartz and fragmentary 

 crystals of feldspar abound, and fragments of a fine-grained light- 

 colored rock may frequently be seen, while darker fragments and 

 pieces of a red or green color may also be occasionally detected. 

 The stratification, while not always apparent in hand specimens, 

 is generally readily observable in the field. Other outcrops than 

 those occurring on the Frowning Ridge escarpment show a remark- 

 ably even bedding or lamination such as is usually only found 

 in material sorted by water currents. It generally forms a hard, 

 well-cemented rock weathering in rough forms. 



The chief interest attaching to this rhyolite tuff is its striking 

 petrographical contrast to the more basic lavas of the section, it 

 being assumed that all the volcanic rocks emanated from practically 

 the same volcanic center. The sequence of the rocks in the section 

 from this point of view becomes of more than local importance, 

 since it bears upon the problem of the differentiation of volcanic 

 magmas, and contributes data to the question as to whether there 

 is or not any definite sequence in the extrusion of lavas which may 

 be regarded as a general law. 



To the south of our line of section both the basic tuff and the 

 doleritic basalt wedge out rapidly, although the lateritic facies of 

 the former continues, and the rhyolite tuff alone persists. Even this 



