Lawson "I 



The Berkeley Hills. 



397 



best seen in the sections afforded by Tumbling Brook, on either 

 side of which it may be traced for a short distance. In the trough 

 formed by the gently-folded andesite lies a remnant of the forma- 

 tion of conglomerate and tuffs which regularly lies above it in the 

 main section of the upper slopes. A glance at the map will indi- 

 cate the disposition of the rocks in this syncline better than any 

 detailed description. No other folds are known to affect these 

 strata; and beyond the edge of the last-mentioned syncline, as 

 marked by the most northeasterly outcrop of the amygdaloidal 

 andesite, we have only southwesterly-clipping strata of the Orindan 

 formation to its base, where it rests on the Monterey on the far 

 side of San Pablo Valley. 



Transverse Fault — In mapping the outcrop of the various 

 sheets of rock, both volcanic and sedimentary, on the northeast 

 slope of San Pablo Ridge, their continuity is found to be distinctly 

 interrupted along a line which runs northerly from the saddle near 

 Eureka Peak. The same strata may, indeed, be found again on the 

 other side of this line, but when the various strata are carefully 

 traced out and plotted accurately upon the map, it becomes very 

 apparent that they have all suffered a dislocation in the same direc- 

 tion along the line indicated. The dislocation, moreover, appears 

 upon the map to be a horizontal one, the strata on the west side of 

 the line being uniformly displaced several hundred feet to the north 

 relatively to the strata on the east side. This appearance of the 

 map is the best possible evidence of a fault, the trend of which is 

 transverse to the strike of the rocks. The fault may have actually 

 differentially displaced the strata, as it appears to do on the map, by 

 a horizontal movement, but a little reflection will show that, inas- 

 much as we are here dealing with the limb of a syncline in which 

 the strata are dipping into the ridge, the same appearance might be 

 effected by a vertical displacement, which would cause a wider 

 portion of the syncline to drop down opposite a narrower lower 

 part. Since vertical displacements on a fault plane are very much 

 more common than horizontal displacements parallel to the strike 

 of the fault plane, it is probable that we are here dealing with 

 a vertical dislocation. The apparent horizontal displacement is, 

 therefore, really the heave of the fault, and, knowing the amount of 



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