1918] Davis: The Badiolarian Cherts of the Franciscan Group 345 



On Presidio Point are to be found beds of a Jaspery rock having a riband- 

 like appearance, and colors from a greenish hue through red-brown to red and 

 yellow; this rock has been considered by Mr. Dana as a variety of the Prasoid 

 rocks, . . . the gradation of prase into jaspery rocks exhibits a close relation 

 of both. 



W. P. Blake, writing to James D. Dana, in 1854, describes the 

 occurrence of quicksilver ore at the New Almaden mine. 103 In his 

 description he says : 



I expected to find the ore in close relation with the deposit of tertiary age, 

 and among bituminous shales, but in this I was disappointed. All the tertiary 

 beds appear to have been washed away, and hard serpentine and trappean rocks 

 constitute a large part of the ridge in which the ore is found. There are however 

 large outcrops of sedimentary rocks, composed of alternating beds of argillaceous 

 shales and layers of flint ; they are highly tilted and much flexed. They are 

 unlike any of the tertiary series that I have seen in the state and I am inclined 

 to refer them to the lower Silurian age. 



A footnote, at the base of the page on which the foregoing de- 

 scription occurs, is as follows : 



Similar rocks (taleose and argillaceous shales and a Jasper rock) occur north 

 of the harbor of San Francisco; they are more or less metamorphic and probably 

 not older than stated by Mr. Blake. J. B. D. 



Blake's first impression of these rocks was such as to make him 

 believe that they were sediments and not metamorphic rocks. In 

 1856, probably influenced by Dana's opinion, he described them as 

 as metamorphic equivalents of the Franciscan sandstone : 100 



The most distinct contortions and highly-inclined positions are, however, 

 shown by a class of rocks which have not yet been mentioned. They are, to all 

 appearance, a metamorphosed or changed portion of the sandstone formation. 

 . . . Portions of the strata are very finely stratified, the layers being not over 

 half an inch thick, and yet they are well defined and apparently very hard. . . . 

 In these islets, and on Lime Point, there are beautiful flexures and folds of 

 the strata, some of them of considerable extent, and others are local, showing- 

 many bends and short angles within the space of a square yard, resembling the 

 compressed and crumpled leaves of a book in the number of thin layers, and their 

 conformity through all the bends. . . . 



The lithological characters of these strata are very interesting. They are 

 hard, flint-like, and jaspery, and occur of various colors. The most common 

 color is a dark reddish-brown, or a chocolate color, but this is often intermixed 

 with yellow and green. Indeed, some of the fragments are beautifully spotted 

 and banded with different colors, and form good specimens of ribbon- jasper or 

 prase. Quartz in thin irregular veins, is a common accompaniment of the rock, 

 and traverses it in all directions without any regularity in the trends of the fis- 

 sures. It appears, in many eases, to form a complete coating around the frag- 



los Am. Jour. Sci., vol. 17, p. 438, 1854. 



106 Pacific Eailroad Rep., vol. 5, p. 155, 1856. 



