GEOLOGICAL REPOKT. 311 



Devonian strata have also been found at several points, and as far west as longi- 

 tude 115° 58', and latitude 39° 53'; that is, '1,200 miles farther westward than they 

 have hitherto been found in situ, as far as it is known to us. We have good reason 

 to believe that they exist also at an intermediate point in the Medicine Bow Mount- 

 ains or their neighborhood. (See section III.) 



The Devonian rocks are also blue limestones and slates, and do not differ essen- 

 tially in their litlu ►logical character from rocks of the Carboniferous formation. A con- 

 siderable development of siliceous conglomerates and sandstones, found at a higher 

 level than the Devonian rocks, apparently occupy the position of the Old Red of the 

 English geologists. 



As yet we have no conclusive evidence of the existence of Silurian strata in this 

 district ; but there is a considerable development of magnesian and siliceous limestones, 

 which circumstantial evidence leads me to consider as belonging to that formation. 

 They contain only a few fossils. Some fragments of tvochifunn univalves, and some 

 coralline forms found in them, do not afford a sufficient criterion, but are not unlike 

 some from Silurian strata of the Mississippi Valley. 



West of 116° of longitude these stratified rocks nearly disappear. Indications of 

 them have been found at various points beyond; but they are so thoroughly altered 

 by the influence of the igneous rocks, that no traces of fossils could be found; nor 

 could I decide whether they are altered beds of the Paleozoic formations, or perhaps 

 of a more distant age. 



No strata of a period more recent than the Paleozoic have been found in the 

 mountain ranges, along our line of exploration, with the exception of some quite recent 

 formations. If they have ever been formed they must have been swept away entirely. 

 Information communicated by Dr. Charles Brewer, United States Army, seems, how- 

 ever, to indicate that more recent, perhaps Triassic or Cretaceous, strata extend into 

 the basin from the east, across the southern continuation of the Wahsatch range. 



No marine Tertiary strata have been observed like those which occur in the south- 

 era lower portion of the basin. All the more recent deposits in the valleys are evi- 

 dently lacustrine and local. 



By the numerous pluto-volcanic eruptions the stratified rocks have been much 

 disturbed. In the single mountains they are tilted in every possible direction and 

 degree.' Their dip is frequently reversed several times within short distances, and 

 great contortions and faults must have been occasioned. Moreover they exhibit a great 

 sameness in appearance throughout, and are generally badly accessible, and only at 

 long intervals. No section could be obtained under these circumstances. The thick- 

 ness of these Paleozoic strata, however, is very considerable. Hundreds of feet have 

 been observed of each one of the formations mentioned above, and the whole must be 

 measured by thousands. 



THE VALLEYS AND THEIR LACUSTRINE FORMATIONS. 



The extensive valleys occupy about half the area of the whole district. Besides 

 some outliers of the igneous and older stratified rocks of the mountains, we find in 

 them indurated strata only at a few points, and these are mostly stratified horizontally, 



