Lawson "I 



I 1 ALACH E J 



The Berkeley Hills. 



the two basins is pertinent to this question. It is that the con- 

 stituent members of the Campan series are much more lenticular 

 than in the Berkeleyan. The interdigitation of these rather thick- 

 lenses would imply of course considerable differences in the 

 strength of the aggregate when subject to stress. The stronger 

 portions of the basin might be flexed, while the weaker portions 

 broke and slipped. The comparatively persistent and regular 

 strata of the Berkeleyan conduced simple flexure. The faulting is 

 most abundant in the Campan basin where the lenticular character 

 of the constituent members is most pronounced. Another con- 

 sideration may be referred to. It is possible that the Berkeleyan 

 series may have been more deeply buried, by other rocks now 

 removed, at the time of its deformation, and so have been con- 

 strained to yield in a plastic manner to deforming stress, while the 

 Campan rocks, being probably nearer the surface at the time of 

 their deformation, would have been more liable to yield to similar 

 stress by fracture. 



It is to be noted, however, that the dominant fault of the region, 

 the Canon fault, is not a compression fault but is a gravity or 

 normal fault. We have exemplified, therefore, in this region, an 

 alternation of compressive stress resulting in flexure and thrusting, 

 and gravitative stress resulting in normal faulting. The cause of 

 this alternation of the direction of stress exemplified in so many 

 disturbed regions is one of the most fundamental problems in 

 orogeny, and still awaits a satisfactory solution. 



It is further worthy of note that during the deformation of 

 the Campan the Berkeleyan syncline acted as a resistant buttress 

 against which the Campan basin was flexed and broken. 



MICROSCOPIC PETROGRAPHY OF THE VOLCANIC 



ROCKS. 



THE ANDESITES. 



The andesitic rocks present a very uniform mineralogical com- 

 position, and may all be classed as pyroxene andesites. Structural 

 variations are, however, both numerous and well defined, and it 



