PEAtE.l GEOLOGY GUNNISON RIVER. 109 



77. Therefore the "western side of the anticlinal fold must change to • 

 a fault. 



If there is no fault, the force of upheaval must have been greater 

 on the eastern side of the Archseau area. If this be so it would ac- 

 count for the fact that the Gunnison Eiver keeps on the eastern side, 

 as seen on the map. We would have to suppose, also, that a portion 

 of the plateau formed an island in the Cretaceous sea. The western 

 side of the plateau will have to be followed carefully before the exact 

 relations can be determiued. This I hope to do during next season. 

 When we were in the Uucompahgre Valley it was late in the season; 

 there was but little water, and, our supplies being reduced, we had to 

 make forced marches, so that we were unable to finish the work on the 

 western side of the canon. 



As already mentioned, the rest of the Archaean area on the Gunnison 

 is very narrow. It is inaccessible in most places, and I am therefore 

 obliged to pass by it with merely this reference. The remainder of the 

 course of the Gunnison is in sedimentary formations, which will be 

 referred to in their appropriate places. 



It will be noticed from the foregoing pages that our district for 1874, 

 unlike that of the preceding year, is entirely destitute of any metamor- 

 j)hic ranges. It is true that the metamorphic rocks on the Eagle Eiver 

 are a continuation of those in the Sawatch range; but this portion of 

 the district forms the boundary, as it were, between the work of the two 

 years. It was really within the limits of one of the districts for 1873, 

 but was left unfinished. On the Gunnison and on the Grand, as we have 

 seen, the metamorphic rocks are shown in caiions where the overlying 

 formations have been cut through. In no other parts of the district are 

 there any rocks of Archaean age. 



