364 



University of California, 



[Vol. 2. 



stopping at present to critically examine these two possibilities to 

 determine which is the correct explanation, it may be remarked 

 that, whichever explanation we adopt, there is implied the super- 

 position of the Monterey formations upon a surface of Chico rocks 

 which had been subjected to the degrading influence of subaerial 

 erosion, so that we have here all the conditions of an unconformity. 

 The recognition of this unconformity is one of the important 

 lessons to be learned in the study of the section. 



Cherts and Shales. — Following the crest of Skyline Ridge, the 

 rocks of the Monterey series are first met with at a point about 

 1,700 feet southwest of the summit of Sugar Loaf. The first indi- 

 cation that we meet with of a passage from one formation to 

 another is the change in the character of the soil; for at the actual 

 contact on the Ridge crest there is no prominent outcrop of rocks 

 of either formation. Instead of the gray or yellowish sandy soil 

 which mantles the Chico rocks, we meet with a light soil free from 

 sand, but abounding in angular fragments of rather hard whitish 

 shale or of a much harder flinty rock, to which the term chert is 

 applied. A few minutes' search on. the steeper portions of the 

 slope or on the summit of the hill which rises abruptly as soon as 

 we meet with these fragments, will reveal ledges or outcropping 

 edges of the beds of this formation. These all show a thinly and 

 evenly-stratified character, with very commonly an alternation of 

 chocolate-colored shale with a flinty dark chert. The white color 

 of the angular fragments found in the soil is due to a bleaching of 

 the rock under the influence of weather and vegetation. Rocks of 

 this character crop out abundantly along Skyline Ridge to a point 

 about 625 feet beyond the summit of Sugar Loaf, and it is evident 

 that we have to deal with a voluminous formation of a remarkable 

 and fairly uniform petrographic character. When, however, we 

 examine the various outcrops critically we find that the cherts vary 

 somewhat in detail. Some beds are quite homogeneous, while 

 others, still thoroughly flinty, show a fine 'laminated structure, the 

 laminae being parallel to the bedding planes. The color of the 

 chert is in some cases coal black, as on crest line at the west base 

 of Sugar Loaf, and in other occurrences it has a purplish cast, 

 although its general appearance, where not bleached, is a dark 



