PEAiE.] GEOLOGY — SILURIAN STEATA. 113 



tached from the Calciferons group, and considered a part of the Pots- 

 dam. jSTot being able to define the boundary between the top of the 

 series and the next succeeding group, I cannot give the thickness. The 

 entire thickness of the Silurian will be given in another place. 



Uagle Biver. — The Calciferons group is mainly shown on the east side 

 of Eagle Eiver, until a point below the caiion is reached, when it crosses 

 to the other side. It does not extend up the slopes as the Potsdam 

 group does, although patches of it may be ibund scattered over it. It is 

 entirely conformable to the* Potsdam group, and of course follows it 

 across the country to the southward. 



Grand Elver. — The only locality on Grand Eiver, within our district, 

 for 1874, where there is any probability of the group occurring, is in the 

 canon between Eagle Eiver and Eoaring Fork. The caiion was not fol- 

 lowed by us, and therefore I cannot positively assert that it is shown 

 there. The reasons for the opinion are given in the preceding chapter 

 ■when speaking of the probability of metamorphic rocks appearing there. 



Gunnison Eiver. — The group occurs nowhere on the Gunnison Eiver 

 from the mouth of Ohio Creek to Grand Eiver. 



REMAINDER OF THE SILURIAN. 



The remainder of the beds that I have included in the Silurian con- 

 sist of limestones, which are for the most part magnesian. In one 

 place on the eastern side of the caiion of Eagle Eiver, there is just 

 above these limestones a bed of trachytic-lookiug rock, 50 or 60 feet in 

 thickness. I was unable to trace its extent, but farther north, between 

 some higher layers, found a bed of similar rock, which was probably 

 derived from the same source. 



The following is the section of the Silurian strata, as exposed on Eagle 

 Eiver : 



1^0. 1. — Section of Silurian strata on Eagle Eiver. 



Top. Thickness. 



( ?) Ft. In. 



7. Light-blnish limestones on Tveathered surfaces, white and yellow. It is 

 in bands of from three to eight inches thickness, w ith a cross frac- 

 ture and non-fossiliferous. A great portion of these limestones are 



probably magnesian. At the toi) they are crystalline 219 6 



Calciferous Group : 



6. Space, in the upper portion of which there is an out-crop of metamor- 

 phosed conglomerate, seemingly composed of pieces of white quartzite 

 and brown sandstone. The masses are irregularly Bhaj)ed. The out- 

 crop is only a few feet in thickness, and the remainder of the space is 

 probably filled with sandstones and quartzites with perhaps a few 

 shales. The space was so covered that the beds were all concealed. 

 The entire thickness is 68 4 



5. Milk-white quartzite, similar to that of No. 1 4 9 



4. Space probably filled with sandstones 22 8 



3. Grayish-brown laminated sandstones with a greenish coating and mud- 

 marks on the surfaces of the laminae 98 6 



2. Fine-grained, rather compact, glauconitic sandstone, somewhat lami- 

 nated, dark brown and greenish-gray 10 



Pnimordial Group : 



1. Milk-white quartzite , 400 



Gneiss. 



Total thickness Silurian about 819 9 



It is probable that a portion of the limestone marked No. 6 in the 

 section above, should be referred to the Trenton, or perhaps to the 

 Niagara group. It is impossible to say whether No. 7 is all Upper 

 Silurian or jiart Lower. 

 8 H 



