108 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



zoisite. It is probable that only ralcareous spots or areas in the shales 

 have yielded zoisite. 



The occurrence of zoisite in the fossiliferoiis phthanites, associated as 

 they are with the other metamorphic rocks, is very significant and aflford.s 

 ample justification, if this were needed, for regarding zoisite as indicative 

 of the metamorphic character of all the rocks in the Coast Ranges, in which 

 it is found under conditions excluding the supposition that it is of later 

 formation than the accompanying minerals. 



In the mass of the phthanite.s included between the cjuartz veins 

 remains of clastic structure are often visible, especially in reflected light. 

 The most interesting constituents of foreign origin are round spots, wdiich 

 often retain evidences of organic chai'acter. Prof Joseph Leidy, at my 

 request, has examined some of the thin sections containing such spots, 

 which he regards as probably foraminiferous shells. 



SERPENTINE. 



Mineraiogicai character. — Scrpeutiue occurs iu irrcgular areas throughout the 

 quicksilver belt, sometimes in comparatively pure masses and sometimes as 

 one of the mineral constituents of altered sandstones and granular, meta- 

 morphic rocks. No exact estimate can be made of the area covered by 

 serpentine, but it is believed to occupy not less than 1,000 square miles 

 between Clear Lake and New Idria. As important results concerning L.e 

 genesis of serpentine and the history of the rocks with which it is connected 

 depend upon the correctness of its identification, a somewhat detailed de 

 scription of its chemical and physical properties is essential to the purposes 

 of this investigation. In studying the collections it was found that, although 

 the physical properties of the hand specimens seemed in many cases clearly 

 indicative of their mineraiogicai character, the microscope revealed such 

 great differences in the optical behavior of the substance supposed to be ser- 

 pentine as to lead to a doubt of its mineraiogicai homogeneity. Such differ- 

 ences might indeed be anticipated from the statements in previous publica- 

 tions. Prof J. D. Dana^ says that in serpentine when pseudomorpliic there 

 is no polarization or only irregular colors as in amorphous or cryptocrys- 



' System of Jriiioralogy, p. -Ifil. 



