796 DESCEIPTIVE GEOLOGY. 



consist principally of silica and alumina, with some alkalies, and a small 

 percentage of iron. It is probably what is generally called seladonite. 

 Microscopical examination of these basalts confirms the conclusions drawn 

 from their external appearance. They are seen to be much altered, the mag- 

 netite is changed into yellow hydrous oxide of iron; there are no distinct oli- 

 vines; the augites are pale and scarce; in the green powder is found titanic 

 iron, which, when fresh, cannot be distinguished from magnetite. The pecu- 

 liar structure of the augite-microlites is shown in Vol. VI, Plate I, fig. 19. 



The highest point in this region, a little north of the bluffs above 

 described, called Hai'din Mountain, is capped by a light mauve-colored 

 rhyolite, containing, in a rather porous felsitic groundmass, crystals of 

 sanidin and quartz, and fragments of white and yellow pumiceous 

 rhyolites. On the eastern flanks of this hill were found other outcrops of 

 decomposed basalt, and in a ravine called Star Canon, running east and 

 west across the mountains, at a considerable distance to the eastward, 

 similar successions of basalt beds dipping eastward, underlaid by a variety 

 of rhyolitic breccias, and in one case overlaid by a reddish, earthy, rhyolitic 

 breccia. A rounded hill on the eastern borders of the mountains, known 

 as Utah Hill, consists of a breccia-mass of hard flinty rhyolites containing 

 free quartz, with very little cementing material between the fragments. 



Among the interesting occurrences in this region is to be mentioned 

 a deposit of pisolite, which is found as an incrustation round a spring in 

 these mountains, about 15 or 20 miles north of the limits of the map. The 

 specimens, brought in from this place by the miners, are aggregations of 

 grains about one-fourth to three-eighths of an inch in diameter, perfectly 

 white, of concentric structure, but having the form of very regular pentago- 

 nal dodecahedrons, which may possibly be a result of contraction or mutual 

 compression. From the Forman Mountains, to the westward, was brought 

 also a curious light-gray rhyolitic rock, interesting on account of its pecu- 

 liar columnar structure, the columns being about an inch in diameter, and 

 made uj) of an aggregation of very perfect little hexagonal prisms about an 

 eighth of an inch in diameter each. The only crystalline ingredients visi- 

 ble are a little free quartz and a few feldspars. 



