GRANITE HILLS. 853 



by a narrow north and south dike of rhyoHte, which, inasmuch as such dikes 

 are of a somewhat rare occurrence in granitic bodies, requires a special 

 detailed description. It is found just below the crest of the ridge, standing 

 almost verticall}^, with the lines of contact between it and the granite-por- 

 phyry sharplj^^ defined, and showing scarcely any indication of alterations 

 or contact-products. A remarkable feature of this rock is its superficial 

 resemblance to the main body of granite-porphyry, which, however, even 

 a cursory examination dispels, revealing a fundamental difference in the 

 structure of the two rocks. This rhyolite varies from light brown to a dark 

 cream color, possesses a characteristic rough texture, and, under the hammer, 

 breaks much more readily and with a totally different fracture from the 

 granite-porphyry. Small but brilliant sanidin feldspars are the only min- 

 eral secretions observable by the unaided eye, which are scattered through 

 a microfelsitic groundmass. To the southward of the rhyolite is a dike of 

 black vesicular basalt, also standing at a high angle, and striking approxi- 

 mately north and south. 



