﻿158 SMITH. 



In discussing those formations it will be best to begin with the one 

 which is stratigraphically the lowest. 



The Jasper Formation (Dungn-Dungan). — This formation was first 

 discovered on the Dungn-Dungan estate, and it has derived its name 

 therefrom. It is perhaps the most interesting of any with which this 

 paper deals. It is exceedingly limited in its outcroppings and quite 

 variable in its phases, never being encountered as a continuous forma- 

 tion, but only as isolated outcrops, which reveal little or nothing as to its 

 position. 



On the left bank of the Baruyen Eiver, about 200 feet up the slope, 

 seemingly projecting out of the talus of a hill which I know to have 

 a serpentine core, is an outcropping of this formation; here it appears 

 to possess more the character of a slate, the fissile slabs varying in 

 thickness from 5 millimeters to several centimeters. It is of a dirty 

 red color, fine grained and compact. The slabs are exceedingly hard, 

 but easily break off with a ringing sound. In the Caraon Eiver this 

 formation is very much brecciated (PI. VII, fig. 9), but the angular 

 fragments have been firmly recemented. Float bowlders were seen in 

 this same stream; they are wholly without structure and in color are a 

 brilliant red, resembling very much the jasper associated with the hema- 

 tite deposits in Michigan and Minnesota in the United States. The 

 resemblance of some phases of this formation to the radiolarian chert of 

 the San Francisco peninsula which I have seen, also led me to make some 

 sections, with the following results : 



DESCRIPTION OF THE SECTIONS. 



In thin section this rock is seen to consist of a fine-grained, amorphous 

 groundmass of chalcedonic silica, copiusly stained with oxide of iron, 

 with almost innumerable round and oval areas which are more or less 

 clear. (PI. IX, fig. 10.) In ordinary light the whole section resembles 

 sections of some of the radiolarian cherts of the San Francisco penin- 

 sula, 10 to judge from my memory of them and from descriptions. 



Between crossed nicols these areas are seen to be filled with a doubly 

 refracting material which often exhibits undulating extinction, and 

 which is in a more or less granulated condition ; by using a higher power 

 (number 4 objective, 3 ocular), it is clearly evident that this granulated 

 material, with every optical character of chalcedonic silica, constitutes 

 both the groundmass and the clear areas. (PI. VIII, fig. 11.) 



I quote somewhat at length from Mr. Lawson's paper X1 because of 



10 Lawson, A. C. : Geology of the San Francisco Peninsula, 15 Ann. Rept. U. S. 

 G. S., 420-426. 



II Loc. cit. 



