1918] Davis: The Radiolarian Cherts of the Franciscan Group 347 



one of the most peculiar features of the mountain, and the material of which the 

 culminating point itself is made up. On the north side of the North Peak, these 

 beds are finely exposed, forming a lenticular mass about two miles long and half 

 a mile wide. They have a nearly east and west strike and dip to the north. 

 They are here, as elsewhere, of a red color, varying from a dull brick-red to a 

 brilliant vermilion hue. The strata are usually thin, an inch being about then- 

 average thickness, and they are much folded together and twisted. These jaspery 

 strata on the north side of the North Peak do not extend around so as to pass 

 to the north of the Eagle Point Ridge, but may be traced in the ravines in which 

 Bagley Creek heads, passing into the unaltered shales of undoubted Cretaceous 

 age in winch Ammonites, Inoccramiis, and other fossils have been found, and 

 which are largely developed to the north of the mountain as well as to the south. 

 . . . No one making an examination of this part of the mountain could doubt that 

 these jaspers are the result of the alteration of the Cretaceous shales. 



. . . The red and green jaspery rocks, however, are the most characteristic 

 forms, and having been here so unmistakably traced to their origin as Cretaceous 

 shales, they have been of great service to us in recognizing this formation in 

 other localities, where the facilities for tracing it out in all its connections, and of 

 determining its age by fossils were less than they were found to be in this vicinity. 



Whitney regarded his ideas as to the metamorphic nature of the 

 various rocks of the Franciscan and their Cretaceous age, as proved 

 by the finding of an Inoceramus in the rocks of Angel Island in the 

 Bay of San Francisco. 



When we consider certain facts regarding Whitney's work it is 

 not surprising that he reached this conclusion regarding the origin of 

 the cherts. Whitney, before coming to California, had worked in the 

 Lake Superior region among Archean rocks. He had seen the fer- 

 ruginous jaspers of that region classed as metamorphic rocks and it 

 is but natural that when he encountered somewhat similar jaspers at 

 Mount Diablo he should have regarded them as similar in origin to 

 the occurrences in the region of Lake Superior. He was familiar, 

 also, with the fact of the occurrence of similar "ribboned jaspers" in 

 the Ural Mountains, and knew that they had been explained there as 

 being highly silicified schists. 110 



At Mount Diablo, Whitney found jaspers and other rocks of 

 Franciscan age resting above Knoxville shales as a result of displace- 

 ment along an overthrust fault. Since he did not recognize the nature 

 of this structure, it is only to be expected that, with his experience 

 of similar rocks elsewhere, he should believe that the upper part of 

 the mountain represented the exposure of a core of metamorphic rocks, 

 and believe that the rocks around the base represented their unmeta- 

 morphosed equivalents. 



no Foster and Whitney, Report on Lake Superior, pt. II, U. S. Sen. Exec. Doc. 

 no. 4, p. 3, 1851. 



