60 EXPLORING EXPEDITION FKOM SANTA FE 



WASATCH MOUNTAINS. 



At present it is impossible to say what is the extent, or very definitely what 

 is the structure of the great mountain groups bordering the Great Salt Lake and 

 extending thence southward toward the Colorado. Of these the more conspicuous 

 ranges, lying east and south of the lake, have been designated by the name of 

 Wasatch. From the explorations of Captains Gunnison, Whipple, and Fremont, we 

 learn that a series of mountain ranges extend south and west from these to and be- 

 yond the Vegas de Santa Clara. In this region they form the northwestern boundary 

 of the Colorado plateau, holding the same relation to that great area of stratified 

 rocks as do the Aquarius and Aztec ranges farther south. Their flanks are covered 

 with Carboniferous limestones, turned up and broken through by their granitic axis. 

 How broad the mountain belt may be which exhibits this character we do not yet 

 definitely know. We are informed, however, that there are several distinct ranges 

 having a trend north and south, or a little east of north by west of south. Going 

 westward these ranges merge insensibly in the "lost mountains" of the Great Basin, 

 some of which retain, in a greater or less degree, the structure of the Wasatch ranges, 

 while others are composed mainly of trappean rocks, having the composition and 

 ragged, picturesque outlines of the cockscomb sierras of the Lower Colorado. All this 

 portion of the continent is thickly set with mountain groups or ranges, forming a laby- 

 rinth whose mysteries years of patient effort will hardly suffice to penetrate and reveal. 

 This region has been recently traversed by Captain Simpson, United States Topograph- 

 ical Engineers, with interesting thoiigh as yet unpublished results. Among the collec- 

 tions made by Mr. H. Engelman, the geologist of the party, are a few Devonian fos- 

 sils, such as Atrypa reticularis and several species of Spirifera, described by Mr. Meek 

 (Proc. Acad. Nat. Sci. Phila., July, 1860), the first instance where fossils of this age 

 have been found so far west. 



Up to the present time we have no well-marked geological criteria by which to 

 distinguish the Wasatch ranges from those of the Rocky Mountains, and it is possible 

 that future explorations will show that the Colorado Basin is completely encircled by 

 the widely separated chains of the same vast mountain system. 



SAN FRANCISCO GROUP. 



Of the San Francisco Mountain and its associates, Bill Williams, Mount Ken- 

 drick, and Mount Sitgreaves, a detailed description is given in the seventh chap- 

 ter of my report to Lieutenant Ives, United States Topographical Engineers. By 

 reference to that description and the accompanying geological map it will be seen 

 that these mountains are great volcanic foci, the eruptions from which have cov- 

 ered all the adjacent country with lava. The precise date of the commencement of 

 the volcanic action in this region cannot, perhaps, be accurately determined, but it is evi- 

 dent that the greater portion of the erupted material was thrown out, geologically speak- 

 ing, at a comparatively recent epoch, at a time when the topographical features of that 

 vicinity were in all essential particulars the same as now ; when the high plateau from 

 which these mountains rise was already a plateau elevated several thousand feet above 



