. Report of the geological exploration of the fortieth parallel . ough, reddish, trachytic breccia, above Avhichrise the white rocks from which the locality takes its name. They aredomed masses, about 300 feet high, of grapsh-white quartziferous bosses of rock have such smooth, even sides that they are exceedinglydifficult of access. The rock is a crystalline aggregation of sanidin (theindividuals of which sometimes reach an inch in length), brilliant blackprisms of hornblende, flakes of biotite, and cracked, rounded granules ofquartz. It shows a close resemblance to the family
. Report of the geological exploration of the fortieth parallel . ough, reddish, trachytic breccia, above Avhichrise the white rocks from which the locality takes its name. They aredomed masses, about 300 feet high, of grapsh-white quartziferous bosses of rock have such smooth, even sides that they are exceedinglydifficult of access. The rock is a crystalline aggregation of sanidin (theindividuals of which sometimes reach an inch in length), brilliant blackprisms of hornblende, flakes of biotite, and cracked, rounded granules ofquartz. It shows a close resemblance to the family of quartziferoustrachytes of It would seem that all the trachytes of the Salt Lake region naturallygroup themselves into two main systems of eruption—the great body eastof the Wahsatch, with its northern continuation, which marks one of theorographical faults of the Wahsatch; and that of the Traverse and Aquimountains and Cedar Range, which, though irregular in trend, is practi-cally at right angles to the first-named series. GesIEzpI 40Parallel. < TRACHYTES. 595 Over the whole broad desert lying to the north and west there are notrachytes, with the exception of a small body on Peoquop Creek, in thenorthern part of the eastern half of Map IV. Peoquop Creek drainsthrough Thousand Spring Valley a few miles north of the Pacific Railroad,and traverses a low region of which the geology is quite simple. It consistsof island-like spurs and hills of Weber quartzite, surrounded and overlaidby horizontal strata of Pliocene. Thi-ough the quartzites has outpoured asmall body of trachyte, over and around which the Pliocene strata havebeen deposited nonconformably. It forms a long north-and-south ridge,with several dome-like points. The rock is more or less decomposed, andis characterized by pores and cavities filled with both calcite and chal-cedony. It is made up of sanidin, plagioclase, and hornblende. The twofeldspars are present in about equal proportion, and the rock
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