DIKES IIS T CRANDALL BASIiS T . 225 



and is deeply furrowed by lateral gulches with rocky spurs, whose upper 

 slopes are thinly clad. The geological structure of this ridge is specially 

 significant and was carefully studied. The breccia composing it at its 

 southern base is basaltic and scoriaceous, with massive flows of vesicular 

 basalt of the same petrographical habit. On one of its southern spurs, 

 about 5h miles from its eastern end, a number of dikes trend N. 70° E. 

 Near the top of the ridge two trend N. 80° E., and one trends east. They 

 vary in width from 18 inches to 8 feet. A narrow dike near the top trends 

 S. 10° E. 



On the crest of the ridge, about a mile and a half from its western 

 end, there is a light-gray indurated tuff, in places containing small frag- 

 ments of rock and carrying some plant remains. The northern face of the 

 ridge at this point is an almost vertical precipice, exposing breccia without 

 a trace of bedding, utterly chaotic, slaggy, and scoriaceous, containing large 

 fragments of massive basalt. In some places it is composed of small angular 

 fragments; in others it is brightly colored, and is cut by dikes which trend 

 a little north of east, very nearly parallel to the crest of the ridge. 



About a mile east of this point the breccia consists of vesicular basalt, 

 with very large feldspar phenocrysts. Some masses of this rock are 20 

 feet in diameter. Near this an amphitheater on the north side of the ridge 

 exposes alternating layers of breccia and lava flows, having a rather steep 

 dip to the southeast. They appear to have been part of a small cone at 

 one time. This part of the ridge is cut by dikes trending a little north of 

 east, and also by a broad dike, 10 feet wide, trending north and south, with 

 a steep hade to the west. Farther east on the crest of the ridge the breccia 

 becomes indurated and weathers into small fragments. It continues to be 

 indurated eastward as far as explored. It is traversed by dikes trending 

 N. 50° E., and by one dike, 18 feet wide, running S. 85° E. Farther east 

 there are other dikes cutting the ridge in a northeasterly direction and 

 trending toward the two deep gulches on Hurricane Mesa, on the northern 

 side of Closed Creek branch of Crandall Creek. Some of the dikes are 

 narrow, but several are quite large, one being 10 feet and another 18 feet 

 wide. Along the portion of the ridge explored by the writer, a distance 

 of about 4 miles, there are 31 dikes; of these 26 are basalt and 5 are 

 hornblende-mica-andesite. They are not uniformly distributed along the 

 crest of the ridge, but occur in groups of from 3 to 8, the largest groups 



MON XXXII. PT II 15 



