GRASS CANON. 785 



pearlite, in contact with which the white frothy matrix is seen to be com- 

 pressed and hardened, so that the surface of the cavities left by these frag- 

 ments is smooth and hard hke a plaster mold. 



The obsidian balls, which have an almost perfectly spherical shape, and 

 occur- imbedded in a layer of pearlite near the summit of the saddle, are 

 seen by microscopical examination to be remarkably pure, containing only 

 a few trichites in a light-gray glass. 



About a mile from the mouth of Grass Canon occurs another white 

 rhyolitic tufa or breccia, of much more compact mass than the above, and 

 enclosing fragments of dark porphyritic rhyolite with free quartz, which 

 forms quite high cliffs on the west wall of the canon. 



In this vicinity, also, is a considerable development of basa:ltic rocks, 

 which have apparently poured out on the east side of the canon, and have 

 also covered the upper part of the ridge on the west. These basalts develop 

 a columnar structure, particularly on the slopes of the peak on the east side, 

 which has been called Basalt Peak, where they are remarkably perfect and 

 arranged horizontally. They belong to the same general type as those of 

 Aloha Peak. The main mass is a compact, dark, rather vitreous-looking 

 rock, with conchoidal fracture and somewhat coarse texture, in which only 

 small plagioclase crystals can be detected macroscopically. The microscope 

 detects also olivine and augite, and in the groundmass an amorphous globu- 

 litic base. 



The basalt from the very summit of Basalt Peak is more crystal- 

 line, showing, under the microscope, plagioclase, green augite, olivine, and 

 magnetite, with a little brown, slightly globulitic glass. To the south of Ba- 

 salt Peak, east of the head of Grass Canon, and apparently underlying the 

 basalt which forms the summit of the peak, is a considerable thickness of 

 basaltic tufa, which macroscopically resembles the palagonitic tufas. It is 

 very soft and crumbly, being simply a loose aggregation of finer or coarser 

 particles of volcanic ash. Under the microscope, it is seen to be composed 

 of splinters of brown glass, with a few fragments of brown augite and color- 

 less feldspar. As the glass does not gelatinize after treatment with acid. 

 Professor Zirkel proposes for it the name of hyalomelane-tufa. 



Basalt forms the eastern wall of Grass Canon throughout the greater 



50 D G 



