CHAPTER XI. 

 RECENT BASALTS. 



By Joseph Paxson Iddings. 



The recent basalts are those that were erupted after the extravasation 

 of the rhyolite or during the period of rhyolitic eruptions. They are in 

 most cases distinguishable from those basalts that were associated with the 

 early andesitic breccias, and which have been described in connection with 

 those breccias. They occur immediately overlying the rhyolite, or inter- 

 calated between rhyolite sheets, or directly beneath them, in such a manner 

 as plainly to be closely connected with them in respect of the time of 

 eruption. In many cases the basalt has been partially removed by erosion, 

 leaving isolated patches and sheets scattered over the rhyolitic plateau. 

 Remnants of once extensive sheets of recent basalt occur west of the Gal- 

 latin Mountains and The Crags, overlying rhyolite. They are scattered 

 over the plateau south and occur in the broad valley of the Madison River. 

 These basalts are characterized by the scarcity or total absence of noticeable 

 phenocrysts. They are mostly dark gray and porous or minutely vesicular. 

 They are extremely fresh and unaltered, and the feldspars, when large 

 enough to be seen megascopically, are brilliant microtine. Phenocrysts of 

 microtine are sometimes scattered sparsely through the rocks. In a very 

 few instances rounded grains of quartz were observed. 



In a gray porous basalt with multitudes of minute feldspars and 

 numerous brilliant phenocrysts of microtine, which is found on the ridge 

 on the northern side of Madison Plateau (589), there is a narrow streak 

 through the basalt of vesicular and perlitic glass, which is mottled white 

 and dark gray. It is apparently rhyolitic, and is probably the result of a 

 fusion of the two rocks, resembling the occurreuce of intermingled basalt 



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