[GNEOUS FOEMATIONS. 315 



tin- cement than any other substance. By tests made upon brown semituff 

 from various plates on the plains, it appears thai a very large amount is 

 sometimes Soluble in hydrochloric acid, with gelatinization indicating the 

 zeolitic character of the main substance dissolved. In one case 54.59 per 

 cent was dissolved. 



4.NDESITIC PEBBLES OF CONGLOMEBATES. 



The eruptive materials of the Denver beds represented by tuffs and 

 by pebbles of conglomerates form one of the most characteristic features of 

 that formation. It has been shown thai a large part of the series is almost 

 exclusively made up of these materials, and they clearly represent a very 

 large amount of rock destroy ed. The source of these pebbles is not known, 

 and they give the only known clues as to the events of one of the important 

 epochs in the geological history of this region. 



As pebbles of unknown origin, they an- not worthy of detailed 

 description, but it is desirable to record their character with some defmite- 

 ness, because this information makes evident the extent of the volcanic 

 eruptions of the time between the Arapahoe and Denver epochs, a period 

 which seems to have been characterized l>v similar outbursts along the 

 whole Rocky Mountain region from New Mexico to .Montana. 



Mineraiogicai composition. — Examination of hundreds of pehhles in the Denver 



beds, from the bottom to the top of the section, has not definitely revealed 

 the presence of any other eruptive rock type than andesite, hut the mem- 

 bers of this large family are nearly all represented At the siliceous 

 end of the series is a light-colored rock, with sparing phenocrysts of augite 

 and plagioclase in a microlitic groundmass of thoroughly trachytic habit. 

 It is possible that this rock might properly he called a trachyte, but only 

 a single pebble was observed. 



Mica-andesite, generally rich in tridymite or quartz, is abundant in the 

 lower beds of Table Mountain. These and some of the more siliceous 

 hornblende-mica rocks may be called dacites. 



Hornblende and mica are present in varying amounts in manypebbles, 

 and augite is often associated with them in subordinate degree. Classify- 

 ing andesites by the relative abundance of the constituents biotite, horn- 

 blende, and pyroxene, there are all manner of varieties to be found in the 



