GEOLOGICAL REPORT. 21 • 7 



series; in other words, whether they correspond to the Lower Jurassic or Triassie 

 formations which appear to be considerably developed farther south, or to tlu.se much 

 more recent strata which we have observed before in the canon of White Clay Creek. 

 and on Weber River above Echo Creek. The observations in the field were not quite 

 decisive on that point, and the presence of both formations mav be accounted for 

 with some degree of plausibility: but the weight of evidence is rather in favor of the 

 more recent age of these rocks. Apparently, the same strata are prominent south 

 from there, and at a much lower level, on the Red Fork of Uintah River, which from 

 these has received its name. 



The following is an enumeration of the strata which were observed along Potts 1 

 Creek, in descending order, and, although necessarily incomplete as a section, it shows 

 the general character of the formation : 



1. Several hundred feet of mostly red sandstones and conglomerates, and red are- 

 naceous and argillaceous shales, with perhaps some strata of limestones. Not well 



2. White, hard sand-rock, only exposed in a short outcrop. 



3. Dark red friable sandstone. 



4. Some gray slate, mostly argillaceous. 



Farther down the creek the lower strata are better exposed, and we find: 



5. A considerable thickness of mostly light reddish sandstones, but also white 

 ones. 



6. White calcareous shales and slates, and some limestones, some of which are 

 fine-grained with an even fracture, others of an oolitic structure. They contain numer- 

 ous traces of fossils. I obtained there some joints of Pentacrimts, and fragments of 

 Pecten and Ostrea, which indicate that this rock belongs to the Jurassic age. 



7. Light reddish quartzose, not very hard sandstones, probably several hundred 

 feet thick. In an interstratification of finer material I observed numerous Gasteropoda, 

 but their generic characters were obliterated. 



8. Strata of quartzose sandstone, varying in color from white to red. and of differ- 

 ent degrees of hardness, several hundred feet thick. At the junction of Potts' Creek 

 and Duchesne Fork they form high precipitous bluffs, and are then- mostly white and 

 exceedingly hard, and eome of them contain a large percentage of lime. 



These are the lowest strata observed on this river. Continuing down Duchesne 

 Fork we change our course more to the south and southeast, in which direction the 

 strata dip, and we pass them, therefore, in reversed order. 1 observed successively 

 Nos. 8, 7, and 6. Then followed for several miles, partly corresponding to No. f>, and, 

 perhaps, also to the higher numbers, more loose shaly strata of mostly white color, alter- 

 nations of generally arenaceous shales, and shaly sandstones, with some more promi- 

 nent strata of white' sandstone, which series reminded me much of some rocks on White 

 Clay Creek and Weber River, and are most probably the same. They are succeeded 

 by a great thickness of white and brick-red sandstones, with much less shaly portions. 

 Where the Spanish trail comes in, we find heavy beds of white soft quartzose sand- 

 stone, with only thin intercalations of shales, some of which are red. The river here 

 makes a bend to the east, parallel to the trend of these strata, which therefore con- 

 38bu 



