RHYOLITES AND 0ACITES. 47 



these mountains columnar jointing, small gaping cracks caused by the stretching 

 of the nearly cooled lava, caves formed by the collapse of highly vesicular lava, 

 platy structure or parting parallel to the contacts, and other interesting volcanic 

 phenomena. 



Faulting due to rougher dacite eruptions. The Brougher dacite is confined to 

 the southern half of the area mapped. This general dacite area is also coextensive 

 with the region of observed complicated faulting, and a connection between the 

 dacite intrusion and the faulting is suggested. The faulting occurred subsequent 

 to the eruption of all the rocks older than this dacite, while the dacite is unaffected 

 by it. This complexly faulted southern half of the area is also downsunken in 

 comparison with the little-faulted northern portion. Near the dacite necks the 

 observed faults are rather more numerous than elsewhere, and in many instances 

 the blocks adjacent to the dacite have been downsunken in reference to blocks 

 farther awav (PI. VII). From these intrusive necks the faults run in a roughly 

 radiating fashion and seem to follow no regular system of trend (PI. VIII). 

 Detailed study of the contact phenomena of the dacite shows that the minute 

 faults in the tuffs at these points generally have their downthrown side next the 

 dacite. 



From these facts the following conclusions have been reached. The faulting 

 was chieflv initiated by the intrusion of the massive dacite necks (the rhyolite 

 necks were probably not so bulky)." After this intrusion and subsequent eruption 

 there was a collapse and a sinking at the vents. As the still liquid lava sank it 

 dragged downward the adjacent blocks of the intruded rock, accentuating the 

 faults and causing the described phenomena of downfaulting in the vicinity of the 

 dacite. 



In reference to this phenomenon of subsidence around volcanic vents Scrope* 

 wrote: 



"It would appear, however, that in some cases the eruption of volcanic matter 

 is accompanied by the subsidence not only of the column of lava which had risen 

 within the vent, but also of the neighboring surface rocks themselves. Several of 

 the cinder cones of New Zealand, as described by Mr. Heaphy, have been thrown 

 up on a line of fault in the Tertiary strata whose upcast forms the sea cliff, and 

 show a clear synclinal depression of the elsewhere horizontal beds, on either side 

 toward the eruptive vent.'' 



Tuff dikes near contacts. At some points along the contact of the Butler 

 Mountain neck with the Siebert tuffs, particularly on the south and east sides, sand 

 and tuff dikes are observed. They are composed of yellow tuff and included frag- 



a In the North Star and Desert Queen mine workings, along the southeastern part of Mount Oddie, for example, the 

 dip of the lower contact of the rhyolite into the mountain is very flat. 

 >> Volcanoes, p. 225. 



