HOLMES.] AMETHYST MOUNTAIN. 51 



although they are much alike in character and materials, being composed 

 greatly of unworn fragments of eruptive rocks. There is, first, the group 

 of strata occuring in the vicinity of Junction Valley, in the walls of the 

 canon beneath the great rhyolite flows, and which form the lower part 

 of the strata of Amethyst Mountain. They are undoubtedly the oldest 

 of the Tertiary rocks of this region. They are older than the flow of the 

 rhyolites and basalts, and contain, according to Lesquereux, Eocene flora. 

 To all appearances they are truly sedimentary in their character. They 

 contain, so far as I know, but few fossil trees. 



The second is a more widely distributed group that contains pretty 

 ■well but irregularly stratified materials, most of which are coarse and 

 irregular, and are chiefly andesitic, trachytic, and basaltic. In the 

 East Fork district they reach a thickness of 3,000 or 4,000 feet, and cover 

 large areas. They lie apparently conformably upon the earlier groups, 

 and the precise horizon where the first ends and the second begins is 

 not determined. In many cases they appear to rest upon the rhyolites 

 of the plateau, and where found in contact with the old volcanic ranges 

 abut horizontally against them. They contain great numbers of the re- 

 mains of Miocene and Pliocene forests. In many places they have the 

 appearance of ordinary sedimentary rocks, but more frequently present 

 the appearance of alluvial drift or of direct subaerial deposits. The 

 abundant occurrences of forest would find a more ready and consistent 

 explanation if the latter idea were adopted. Captain Dutton has spent 

 a number of seasons in the study of a great series of similar formations 

 in the plateau regions of Utah, and has come to the conclusion that 

 they are wholly of alluvial origin. He presents the matter in a clear and 

 forcible light in his valuable report on the plateau region of Utah. 



The third group forms the cores of the great volcanic ranges around 

 which the horizontal beds are built, and from which much of their 

 material was derived. I have observed rocks of this group both in the 

 Washburn Mountains and in the great range east of the lake. 



The two days to which our visit to the valley of the East Fork was 

 limited, were spent mostly in the examination of Amethyst Mountain. 

 On the third day I crossed to the valley of Soda BuLte Creek, and thence 

 up the river to the first large tributary that enters from the east. The 

 valleys are terraced and drift-covered, and the slopes of the mountains 

 are composed of the usual volcanic conglomerates. 



Camp was made in a high valley near the head of the second stream 

 east of Amethyst Mountain. The rocks are unusually coarse conglom- 

 erates. Some of the massive spurs overlooking the river on the south 

 are capped with heavy sheets of basalt. 



On the following day we crossed the divide of Amethyst ridge and 

 found ourselves on the headwaters of what is probably Broad Creek. It 

 is a remarkable fact that the crest of the ridge is composed of rhyolite, 

 which seems to occur in layers with strong dips in divers directions. In 

 following the divide to the east, I found that while the rhyolites form the 

 crest aud are the chief rocks of the broken plateau which extends south- 

 ward toward the lake and cafion, the northern slopes of the ridge are 

 composed of basalts and conglomerates. The elevation here is generally 

 about 9,000 feet. There is considerable conglomerate debris scattered 

 over the surface of the rhyolites of the plateau. In the region of the 

 divide lakes, which were visited by the survey in 1871, the formations 

 are all rhyolite. These are light in color, distinctly bedded, and have 

 very numerous and large sanidins and quartzes. 



The abrupt basaltic promontories bordering the deep valley of East 

 Fork on the south afford a fine view of the country about the head- 



