64 



University of California Publications. [Geology 



These shales, in the vicinity of Shale Mountain near Chit- 

 tenden, average 76 per cent, in silica, though many of the 

 beds run much higher than this as the following analysis will 

 show : 



Analysis of Monterey Shale from Near Chittenden. 



Per cent. 



Si0 2 87.02 



Al,() :i 2.97 



CaO 1.12 



MgO 25 



Loss on ignition 7.40 



98.76 



Only two species of fossils were recognized, Pecten peckhami 

 and Tellina congesta, the former being exceedingly abundant. 



The Monterey shale rests upon the diorite in the southwestern 

 part, and upon the Franciscan rocks in the northern part of 

 the area mapped. In the sedimentary basin between the two 

 contacts, the shale formation has been compressed into four 

 synelines and three anticlines, all with general northwest- 

 southeast axes. At both contacts the shale overlaps the beds 

 mapped as Lower Miocene and hence these latter beds appear 

 only at the surface in the two central eroded anticlines. Again 

 here as with the Franciscan rocks, these structural features are 

 general. "Within smaller limits the structure becomes exceed- 

 ingly complicated. Some of the complications seem, however, 

 to hold good over the entire area. The anticlines are as a rule 

 simple, while the bases of the synelines exhibit close contorted 

 foldings. This is best seen near the Pajaro River at the Southern 

 Pacific Railroad bridge. Here, at the base of the syncline 

 the beds are twisted into a number of overthrust folds 

 which gradually die out to the northeast and to the south- 

 west where the strikes and dips become fairly uniform. This 

 state of affairs is to be expected, however, where a thick, uniform, 

 sedimentary unit is uplifted and folded. The anticlinal portions 

 of the terrane have practically little vertical resistance to over- 

 come while the synclinal portions are resisted by the underlying 

 rocks and therefore have less vertical compensation. The result 

 is, naturally, intense folding in the synelines. 



