CHAPTER IV. 



STRATIGRAPHY — ARCHAEAN AREAS OF EAGLE, GRAND, AND GUNNISON 



RIVERS. 



As will, perhaps, be evident from the preceding chapters, the greater 

 portion of the country comprised in our district is underlaid with rocks of 

 Tertiary and Cretaceous age, the older formations showing only where 

 there are abrupt folds, and where the streams have cut through the 

 more modern beds to them. 



The other formations represented are the Jurassic, Triassic (?), Carbo- 

 niferous, and Silurian. These will be referred to in subsequent chap- 

 ters. The present chapter will be devoted to the consideration of the 

 Archaean rocks, while separate chapters will be given to the sediment- 

 ary formations, and to the eruptive rocks of the district. 



The Archaean areasare limited, and will be considered in the geographi- 

 cal order followed in the chapters on the general topographical and ge- 

 ological features of the district. 



On account of the rapid and extended character of our explorations, 

 and also the difficulty of getting at these rocks from their being cut into 

 deep gorges by the streams, I am unable to present but few lithologi- 

 cal details. As far as we were able to determine, the rocks are all meta- 

 inorphic, dark micaceous schists prevailing. Until they are studied in 

 more detail, their exact age must remain undetermined, although the 

 occurrence of the Potsdam sandstone resting on them near the head of 

 Eagle River, and at various localities in the district of 1873, proves them 

 to be at least of Pre-Potsdam age. No facts were obtaiued in regard to 

 their relations to the metamorphic series exposed in the Front range. 



Eagle River. — The metamorphic rocks through which the upper tribu- 

 taries of Eagle River cut their courses form the northern extension of 

 the Archaean area of the Sawatch range. This area was described in 

 last year's (1873) report, and therefore I will do little more than refer to 

 it here. The group of peaks of which the Holy Cross Mountain forms 

 one of the most prominent, marks the northern end of the great Sa- 

 watch anticlinal. The sedimentary beds curve gradually around the 

 end of the range, the line of outcrop across which Eagle River cuts, on 

 its way to the Grand being the direct prolongation of the line of out- 

 crop crossing Frving-Pan Creek (noticed in the report for 1873, page 

 266). 



The upper portion of the Eagle is for the most part in cailon cut in 

 these rocks. The caiion is one mainly of erosion. As far as the sedi- 

 mentary beds are concerned it is mouocliual. They, however, are not 

 well shown until we reach the lower part of the canon, which ends a 

 r short distance above the mouth of Roche- Moutonn^e Creek. 



This canon, which has beeu partially described in a preceding portion 

 of the report, is about two thousand feet deep, and presents all the pe- 

 culiarities of gorges cut in gueissic or granitic rocks. The walls are 

 steep and rugged, the river occupying the entire width of the caiion at 

 the bottom. The trail keeps high up on the hills on the eastern side of 

 the river. 

 106 



