t548 DESCRIPTIVE GEOLOGY. 



made up of twisted bands of pale-green glass, traversed by stripes of col- 

 orless glass, both straight and curved. The green glass is composed entirely 

 '.>f pale-green microlites, which are longitudinally arranged and almost con- 

 tinuous through the length of the band. The colorless glass contains both 

 monocllnic and trichnic feldspars, and quartz crystals, which have as enclos- 

 ures apatite crystals containing glass-inclusions. In the glassy material of 

 which this rock is made up, no less than six different varieties of volcanic 

 glass can often be distinguished at once. 



To the west of these breccias is a body of pinkish porphyritic rhyolite, 

 containing a very large proportion of crystals of quartz and sanidin-feld- 

 spar. Under the microscope, the quartz is seen to be full of well-defined 

 glass-inclusions, and the crystals of sanidin, which are unusually clear and 

 free from cracked or foreign interpositions, show the most remarkable blue 

 colors, like the labradorizing feldspars from Frederilssvarn in South Norway. 

 This phenomenon is a comparatively common occurrence in the rhyolites of 

 this region. The blue color is even more intense than that of the Norwe- 

 gian occurrence ; but in the latter the peculiar shimmering light is due to 

 the presence of strange bodies interposed between the laminse of the feld- 

 spar. In these, however, the sanidins are quite free from interpositions, and 

 the phenomenon cannot be explained on this ground. In transmitted light, 

 these sections are quite colorless, and are therefore not easily distinguish- 

 able from quartz, but the quartz is always characterized by dihexahedral 

 glass-inclusions. 



At the extreme western end of the caiionis a mass of green rhyolitic brec- 

 cia, not unlike that of the Mount Airy Hills, but remarkably rich in crystals of 

 quartz, which form at least a third of the whole mass, and constitute almost 

 the only macroscopical crystalline ingredients. The surface of the rock is 

 covered by curious botryoidal concretions of opaline quartz, which also line 

 the cavities of the rock. Under the microscope, the groundmass is seen 

 to differ from that of most of the rhyolites of the Fortieth Parallel, in that it is 

 non-polarizing, and made up of accumulations of small globules which are 

 concentric but not fibrous, the whole being traversed by axially fibrous 

 strings. These accumulations of globules have been named by Vogelsang 

 " cumulites ". 



