CHAPTER Y. 



STRATIGRAPHY — PALEOZOIC FORMATIONS. 



This chapter will be devoted to the Paleozoic areas of the district. 

 They are as a rule few in number and of small extent. As in the case 

 of the metamorphic rocks, the Paleozoic formations are found mainly 

 along the courses of the great arteries of the district, never extending 

 any great distance from them. They appear there outcropping beneath 

 the rocks of Mesozoic age which cover a large part of the region. They 

 have been identified principally by their position and lithological char- 

 acters. Although frequent and careful search was made for fossils, it 

 was generally without success. In Mr. Marvine's district organic remains 

 were found in more abundance, especially in Carboniferous layers. 



I shall take them up consecutively, commencing with the Silurian, and 

 considering them in the geographical order adopted in the preceding 

 chapters. On the Gunnison Eiver no sedimentary formations older than 

 the Eed Beds (Triassic ?) are seen. At no point in the district was I able 

 to get the entire thickness of the Paleozoic strata. It is, however, proba- 

 bly about 4,800 feet. 



SILURIAN AGE. 



Although no fossils were found by me this year in the strata I shall 

 refer to this age, still I think their position and lithological characters 

 warrant such a reference. They rest immediately on the metamorphic 

 rocks mentioned in the last chapter. We can say definitely that 

 they are of a Pre-Carboniferous age. The upper portion of the series 

 may at some future day, when more data are obtained, have to be con- 

 sidered as Devonian. Fossils of Carboniferous age were found in the lay- 

 ers resting immediately upon them. There was a much greater de- 

 velopment in Mr. Marvine's district, and further details will be found in 

 his report. In my district, the formation is limited to Eagle Eiver with 

 possibly a small area on the Grand. 



lam unable to make any further division of the formation than to 

 separate the Potsdam sandstone from the layers above, the base of the 

 latter being referable to the Calciferous epoch or Quebec group. The 

 entire thickness of the Silurian layers is about 820 feet. 



PRIMORDIAL PERIOD — POTSDAM? GROUP. 



The Potsdam sandstone is widely distributed in the Eocky Mountains, 

 and preserves its characteristics in widely se})arated localities. Dr. 

 Hayden noticed the strata in 1856 or 1857, in the Black Hills of Wyom- 

 ing, and he and Professor Meek, in a paper read before the Academy of 

 lifatural Sciences, of Philadelphia,* announced the discovery of fossils, 



* Proceediugs Academy Natural Sciences, Pliiladelphiaj March, 1858. 

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