334 EXPLOKATIONS ACEOSS THE GREAT BASIN OF UTAH. 



porphyritic varieties. At the upper end of the canon more slates, &c, were observed, 

 but the hills are mostly covered over, and but few rocks exposed. Near the summit 

 I found a trachytic porphyry with a feldspathic matrix and crystals of glassy feldspar 

 and mica, and near by other similar rocks form successive overflows or protrusions, 

 presenting the appearance of stratitication. Some of them reminded me distantly of 

 the rocks of Spring Valley in the Un-go-we-ah range, others of the rocks in Kobah 

 Valley, and one is allied to the pitch-stones. The summit and west side of the pass are 

 composed of granite, and only lower down on the west side some more flint-rock occurs. 

 Where we struck Reese's River, horizontal strata of modern origin were noticed, 

 which must have been formed as lacustrine deposits, partly conglomeratic, partly fine- 

 grained calcareous sandstones, and arenaceous limestones. In the range west of 

 Reese's River, porphyries are largely developed, of mostly light-reddish color, and 

 with crystals of glassy (?) feldspar and mica, and partly of quartz. With them I 

 found some highly altered stratified rocks, especially flint-rock, and a sandstone which 

 had become quite porphyritic by the secretion of crystalline particles of the silex; also 

 sonic black pitch-stone. 



Next follows the Se-day-e range, with subordinate chains. Where it has come 

 under my observation, its main body is nearly altogether composed of plutonic masses, 

 granite, porphyritic rocks, pitch-stones, Ac. White granite was found in the center of 

 the range, near the head of Gibraltar (/anon. Trachytic, and, perhaps, some dioritic 

 porphyries are most largely developed. Their color is generally pink or reddish- 

 brown; others are whitish. Those of the latter, at the mouth of Putnam Canon, 

 exhibit an imperfectly columnar structure. Near the eastern foot of the mountains I 

 noticed various rocks which have evidently erupted at a somewhat later period. 

 There are black and brown pitch-stones, at one place forming a dike, split up by 

 numerous fissures into tabular pieces with glazed surfaces and highly brittle inside; 

 other masses appear as a mixture of the porphyry and pitch-stone, and similar to some 

 lavas; and a large vein is filled with a trachyte which seems to be closely allied to the 

 rock from Weber River, No. 153 of the collection, but contains less quartz and mica. 

 Brown porphyry prevails on the west side, and also in the more western spurs; only 

 in the canons some local tufaceous sediments were observed, and on Edward Creek 

 a flinty conglomerate and some few other ledges of metamorphosed rocks. 



In the park below the Gate of Gibraltar we find extensive deposits of a mostly 

 pure white tufa, apparently formed in a lake which has been drained by the erosion 

 of the Middle Gate. These sediments are formed of finely comminuted trachytic 

 rocks, pumice, &c; and the siliceous shells of Infusoria may have largely contributed 

 to it. They scarcely contain traces of lime. They are apparently identical with 

 those observed by Dr. Newberry on the upper Pitt River, Klamath Lake, &c, and 

 called by him infusorial marls, of which he remarks (Pacific Railroad Report, vol. v4i, 

 p. 39), that they have a stinking resemblance to pulverized pumice, and have doubtless 

 been formed of similar material. I found the same on Carson River, east of Eagle Val- 

 ley, where, however, they contain a few per cent, of lime; but a similar formation f nMn 

 the Salt Lake Desert, near Fish Springs, is a calcareous marl. 



In the Middle and Lower Gates I noticed porphyry, flint-rock, and signs of other 



