108 GEOLOGICAL SURVEY OF THE TERRITORIES. 



) 

 are probably of Cretaceous age, belonging to No. 1 or Dakota group. 

 The reason for this opinion will be given, at length, in a subsequentchap- 

 ter. Farther down the river these sandstones increase in thickness, as 

 shown in Fig. 1, Plate XIV, while the gueissic area is very narrow. 



The presence of the Dakota group, resting immediately on the Ar- 

 chaean rocks, would seem to prove that in Pre-Cretaceous times this Ar- 

 chaean area was above sea-level. What its limits were I am unable to 

 say. It was probably connected with the Sawatch range, a portion of 

 which, as I mentioned in last year's report, was probably above the sea- 

 level during Pre-Cretaceous times. There was a period of gradual sub- 

 sidence. This commenced at least in Pre-Triassic ages; for, as shown in 

 map B, as we approach Smith's Fork there appear, resting on the gneiss 

 and underlying the Dakota group, first, beds of Jurassic age, and then 

 the Red Beds (Triassic ?). 



When the subsequent elevation began it is difficult to say, although 

 it is altogether likely that it was Post-Cretaceous. It was probabij 

 gradual. 



Erosion subsequently modified the original surface. Then followed 

 the period when the breccia and lava was poured out, concealing the 

 underlying formations. Between Lake Creek and Cebolla Creek, there 

 are places where the schists and gneiss seem to be capped with trachytic 

 rock, without any intervening layers; while at lower levels, on some of 

 the small branches of Cebolla Creek, there are shales probably of Upper 

 Cretaceous age, abutting immediately against the gneiss and horizontal 

 in position. Near the mouth of Cebolla Creek the schists seem to dip to 

 southeast. Here they are very coarse, with large masses of quartz and 

 pink feldspar. The mica is silvery (probably Muscovite). The Grand 

 canon of the Gunnison in reality commences at the mouth of Lake Fork, 

 although for nearly a mile it is not very deep. Below the mouth of 

 Cebolla Creek, however, it is between 2,000 and 3,000 feet deep. The 

 gneissic portion, opposite station 78, shown in Fig. 2, Plate VII, is 2,000 

 feet deep. The caiion extends to the mouth of the North Fork. 



The ArchJean area in which this caiion is cut is defined in map B, Fig. 

 2. Plate VII represents a section across it through stations 77 and 

 78, to Cedar Creek, on the line marked A B, on the map B. Fig. 1 in 

 the same plate is a section on the line C D on the same map. 



It will be noticed that the metamorphic rocks form a plateau like mass 

 between the Gunnison Eiver and Cedar Creek. This plateau narrows 

 to the northwestward and ends in sharp, isolated peaks. 



South of station 80 there is, however, as shown on the map, a narrow 

 Archaean belt bordering the river, in which it cuts the deepest j)ortion of 

 its caQou. This belt extends to a point below the mouth of Smith's Fork 

 and forms a bench on each side of the river, as seen in Fig. 1, Plate VII, 

 at a. It will be seen in map B that the Gunnison River keeps on the 

 eastern side of the plateau, and that on the west (Fig. 2, Plate VII, h to 

 i), between it and Cedar Creek the strata are horizontal, abutting against 

 the granitic rock. The age of these beds is probably Upper Cretaceous, 

 while the sandstones under station 77, at d and e in Fig. 1, Plate VII, 

 represent the Dakota group (No. 1) or a portion of it. 



From the uniformity of level of the plateau it would appear that the 

 sedimentary beds once extended over it and have been removed by 

 erosion. In the section Fig. 2, Plate VII, they are shown on both sides 

 of the river. If they once covered the plateau there must be a line 

 of faulting along its western edge, for the level of the Upper Cretaceous 

 beds shown there is below that of the Dakota group under station, 



