304 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



in quartz by dynamic forces. It is, however, in this case the result of 

 aggregate crystallization. Such sections exhibit uniaxial interference 

 crosses which are optically positive. The aggregation appears to be made 

 up of thin hexagonal plates, parallel to the basal pinacoid. The aggrega- 

 tion of plates into spherical masses takes place in tridymite, as is well 

 known. Spherulitic aggregations being only modifications of spherical 

 ones, such structures may be produced by the attempts of thin plates to 

 form spherical clusters. The rods would then be at right angles to the 

 vertical axis of the mineral — that is, the apparent fibers would be parallel 

 to IX, the direction of vibration of the swiftest-traveling ray. The lighter 

 specific gravity of chalcedony, as compared with quartz, would seem to be 

 due to inclusions, probably of hyaline silica, since minute inclusions are 

 observed in thin section, and a, variable percentage of water is found upon 

 chemical analysis. Opal replaces the feldspars in a few of the rocks 

 studied. 



Sometimes the amygdules consist of minute zeolites with very low 

 double refraction and a marked pinacoidal cleavage with parallel extinc- 

 tion. It is probably heulandite. The spaces between these crystals are 

 tilled with a colorless, low-refracting substance, apparently composed of 

 minute spherules or plates, resembling tridymite. But the whole mass is 

 spherulitic, with delicately fibrous rods that are optically positive. The 

 double refraction is low. There seems to be no structural relation between 

 the minute spherules and the spherulitic structure. One appears to be 

 subsequent to the other. The chemical character of this substance was not 

 discovered. Similar mineral with stronger double refraction occurs in 

 other varieties of the rock. Other secondary minerals, probably zeolites, 

 are present in a few cases (1682, 1675). 



BIKES AND SURFICIAE PLOWS. 

 VICINITY OF SYLVAN f>ASS. 



Subsequent to the accumulation of the late acid and basic breccia of 

 the Absaroka Mountains there broke out a series of eruptions whose center 

 of activity was in the vicinity of Sylvan Lake. In this neighborhood the 

 breccias are traversed in all directions by dikes of rocks which have a wide 

 range of composition and which also attained great variety of crystalline 

 structure. 



