I40 WALLACE W. ATWOOD 



is a distinct flow-structure, between scoriaceous zones. The scoria- 

 ceous zone at the base is at least 5 feet thick, and that at the top 

 is about 10 feet thick. At another exposure, 6 feet of compact 

 lava overlie a scoriaceous zone 10 feet thick. Imbedded in the 

 scoriaceous zone, at the base, there are huge blocks of compact lava. 

 Both the scoriaceous and compact portions are porphyritic. The 

 exposures of agglomerate are near the layer of lava, and associated 

 with the surface of the flow. 



Small crystals and fragments of crystals are exceedingly common 

 in the tuff. Probably there is not a square foot on the surface of 

 the tuff, as now exposed, where crystals could not be found. The 

 chief minerals represented are plagioclase feldspars, pyroxenes, and 

 hornblendes. The plagioclases have characteristic striations, they 

 are clear and glassy, and range up to an inch in diameter. The 

 pyroxenes and hornblendes are commonly jet-black, and vary in 

 size up to three-fourths of an inch. The crystals are a part of the 

 fragmental material ejected by the volcano, and were therefore formed 

 in the magma before eruption. The formation of these crystals, as 

 well as those in the angular blocks and in the lava flow, caused an 

 increase in the gaseous pressure in the magma, and according to the 

 suggestion of Chamberlin and Salisbury, may have been an import- 

 ant factor in causing the explosions."^ 



The gases occluded in the andesite and in the pyroxene crystals 

 have been determined by R. T. Chamberlin. One volume of the 

 rock gave 6.37 volumes of gas of the following composition: 



H,S o.oi 



CO, 80.38 



CO 9.02 



CH4 4.72 



H, 1.84 



N2 4.00 



99.97 



One volume of the crystals gave i.ii volumes of gas of the fol- 

 lowing composition : 



I Chamberlin and Salisbury, Text-book on Geology, Vol. I, p. 6i8. 



