of the Gold-fields of California. 323 



In addition to quartz, felspar, and mica, the latter of which is 

 well crystallized and often of a brilliant black colour, this rock 

 encloses titaniferous iron, and occasionally crystals of sphene of a 

 light yellow colour. Magnetic oxide of iron is also present in 

 notable quantities, and consequently imparts this property to the 

 enclosing granite. The granite of the Sierra appears to retain 

 its peculiar characteristics throughout a great portion of its ex- 

 tent ; but towards the southern extremity of the chain hornblende 

 begins to make its appearance as one of the constituents of the 

 rock; the mica at the same time becoming less plentiful, and 

 losing its brilliancy and dark colour. 



Lying between the band of metamorphic slates and the great 

 central granitic mass forming the more elevated portions of 

 the chain, are found various eruptive rocks, such as syenites, 

 diorites, and porphyries. The first of these pass into granites 

 by almost imperceptible gradations, and appear to belong to the 

 same geological period. The diorites, on the contrary, are of 

 more recent origin, and contain neither sphene nor titaniferous 

 iron, although they enclose a considerable amount of magnetic 

 oxide of iron. In many places diorite is observed to traverse the 

 granite in the form of distinct dykes. The porphyries are com- 

 paratively of small extent, and contain crystals showing the 

 usual strise, indicating the presence of plagioclastic felspars. 



Quartz Veins. 



All the crystalline rocks in the vicinity of the sedimentary 

 deposits contain numerous veins of quartz, which traverse the 

 granite and diorites near their point of junction with the slates, 

 and, in addition to gold, enclose crystals of iron pyrites and 

 other metallic sulphides. 



These quartz veins of the crystalline rocks are comprised 

 within a narrow zone running from south to north, along the 

 western flank of the mountains above the great band of meta- 

 morphic slates forming the most productive portion of the gold- 

 region, and extend, in the vicinity of the line of junction, nearly 

 throughout its whole extent. 



The quartz veins of the band of metamorphosed slates occu- 

 pying the western slope of the Sierra, and lower on its flank 

 than the crystalline rocks above described, are numerous and 

 important. They are not, however, by any means equally dis- 

 tributed throughout the region of slates, but are chiefly con- 

 centrated in a belt having a width from east to west of some 

 twelve or fifteen miles, and extending from south to north 

 throughout the whole length of the formation. These veins, 

 for the most part, follow the general direction of the strata 

 in which they are enclosed. This parallelism, however, is not 



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