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augite-tracliyte breccia beds and the compact gray augite-trachyte, which 

 rise in the high bluff wall to the northward. The rhyolites themselves are 

 penetrated by two sets of dikes, one of a nearly black, very compact augite- 

 trachyte, which contains in the micro-crystalline groundmass crystals of one- 

 half inch or more in length of sanidin. According to Professor Zirkel, the 

 groundmass is a crystalline admixture of feldspar and pale brownish-yellow 

 augite, many of the feldspars being striated, yet the prevailing forms are 

 monoclinic. Short black lines of hornblende occur, although sparingly. 

 No olivine was detected. So far as observed, these dikes all dip to the 

 west and strike west of north. 



Where these black trachyte dikes break through the rhyolite, there are 

 contact phenomena of exceeding interest, the latter, and in this instance 

 older, rock being converted into a dark chocolate-brown glassy mass, in which 

 only a few feldspars and biotite flakes still retain their integrity, the product 

 having almost the habit of obsidian. From this extreme form, there is a 

 gradation through 3 feet of less and less glassy product into the normal 

 rhyolite of the adjoining region. Professor ZirkeP has described with some 

 detail the microscopical characteristics of these hyaline varieties, illustrat- 

 ing in Plate YIIl, fig. 4, a remarkable obsidian. But for the fact that on each 

 side of these black trachyte dikes the glassy forms of rhyolite pass gradu- 

 ally into the crystalline varieties, it might not seem conclusive that the dikes 

 had been the agents of this glassy modification; but since the same phe- 

 nomenon is repeated at the contact of several of them, it seems improbable 

 that the form could have existed in the first instance, and the dikes have, by 

 accident, followed the course of the glassy bands. This fusion of rhyolite 

 into glass is no more remarkable in contact with the molten dikes of trachyte 

 than is the fusion of trachytic mountain-points by lightning, and the two 

 phenomena would appear to bear a close relation to each other. In this 

 locality, therefore, occurs a marked exception to the law enunciated by von 

 Richthofen of the sequence of rhyolites over trachytes, for here the trachytes 

 have undoubtedly broken through in dikes and overflowed the earlier rhyo- 

 lites, both in the form of breccia and compact material, in the instance of 

 the gray tabular hills to the north. It is, however, a marked fact that while 



* Microscopical Petrography, vol. vi, 206. 



