344 MINING INDUSTEY. 



North of the mouth of the South Twin River CaQon is a small body of 

 granite, resembling that of San Juan in its brownish hue, which seems here 

 likewise to be due to the smoky condition of its quartz; like the latter it con- 

 tains a very small proportion of mica, but the crystals of feldspar are so small 

 that it is not possible to observe the labradorite-like iridescence of the feld- 

 spar of that rock. This granite passes on the west into a fine-grained rock, 

 occurring in tabulated masses, having a reddish-brown weathered surface, 

 which, to the naked eye, has the appearance of a greenstone, but, under the 

 lowpe, discloses a crystalline structure, in which small crystals of quartz and 

 feldspar are distinguishable. 



Still further south, as far as the Hot Springs, the eastern slopes of the 

 range seem to be composed of metamorphic slates. 



In the narrow gorges of the lower Twin River Canons, the rugged grand- 

 eur of whose cliffs surpasses that of any of the other canons of the range, the 

 prevailing rock is a diorite, in general color from dark-green to black, consist- 

 ing of a dark-green, hornblendic matrix, containing small crystals of white 

 feldspar porphyritically imbedded, and locally a few crystals of limpid quartz, 

 though the mass of the rock is without quartz; a light-green, transparent 

 mineral frequently accompanies the feldspar, which may be an epidote. With 

 unimportant variations in general appearance this rock extends from the pro- 

 pylite to the granite in either canon, having an appearance of regular bedding, 

 and interstratified, at different points, with highly metamorphosed siliceous 

 clay-slates, which predominate near the line of contact with the propylite. 

 The whole mass is much broken and confused, so that no satisfactory idea of 

 the relative positions of these rocks was formed; hence, on the map they have 

 not been distinguished by any distinctive color, but included in the slate belt, 

 which occupies the same relative position further north; the whole mass is 

 evidently metamorphic, as well as the eastern body of Ophir Canon granite, 

 and in no part of the range has the action of metamorphism been carried so 

 far as in this region ; a careful and detailed examination of which would doubt- 

 less afford many interesting developments with regard to the relations of gran- 

 ite to the more recent eruptive rocks, and of these to each other, as well as 

 the action of either on the sedimentary rocks. 



In ascending the canons of the Twin Rivers a similar succession of rocks 



I 



