66 GEOLOGICAL SURVEY OF THE TERRITORIES. * 



below, so that there was a gradation from solid sandstones at the top 

 to homogeneous shales at the base. The section includes the group of 

 strata sometimes called the " transition group " by Dr. Hayden. The 

 horizon is probably that of the upper part of No. 5, Cretaceous. The dip 

 of the beds in this place is toward the northwest 10° to 15°. They seem 

 but slightly affected by the elevation of the main range on the east, or 

 of Treasury Mountain on the south. Farther up the stream, which we 

 shall call Aspen Creek, the dip increases to 45°, and the upturned edges 

 are lodged against the granite, which, by means of a rather complicated 

 fault, has been thrust up past the broken edges of the entire series of 

 earlier sedimentary rocks, bending the edges of the older strata back 

 and driving them into the softer strata above. In the bottom of the 

 creek a small portion of the yellow quartzites are exposed, situated as 

 shown in Section D of the large sheet. This fault would seem to be on 

 the northern continuation of the line of upheaval to which belongs the 

 inverted series observed last year about the southern sources of Eock 

 Creek. Our investigations at that time were extended to within six 

 miles of this point. 



Late in the evening we encamped near timber-line, and on the follow- 

 ing morning climbed the high granite ridge to our left. We soon found 

 ourselves in the very midst of the mountains. Snow Mass and Capitol 

 and Mount Daly rose up magnificently in the east, Sopris stood alone 

 at the north, and many groups of lofty mountains appeared in the 

 southwest. All around us were only bare rock and snow. The whole 

 area is above timber-line, and the sculpture of the mountains is won- 

 derfully striking and picturesque. The long crooked lines of crests are 

 connected by subordinate crests, and these all send out sharp, narrow 

 branching ridges which separate the amphitheater-like heads of the 

 numerous radiating streams. As a rule, these high valleys are wide 

 and the ridges narrow, so that the country presents the appearance, in 

 a rude way, of a giant honeycomb. Sopris is connected with Capitol 

 by the fiat ridge of Carboniferous red beds, Capitol with Snow Mass by 

 a deeply-indented saddle, while south from Snow Mass, the axis crest 

 continues to Maroon Mountain, thence to the White Eock and Castle 

 peaks. From the saddle, midway between Capitol and Snow Mass, a 

 pinnacled ridge extends to the westward between the head- waters of 

 East Fork or Avalanche Creek and Eock Creek proper. Branches are 

 thrown out from this between all the small streams, while the chief 

 crest of the spur continues out to station 22. Stations 24 and 25 were 

 made- about midway on this ridge. Station 24 is the most northerly 

 summit of the Snow Mass granite, and is eight miles from station 22. 

 The sedimentary outcrops, which pass just north of Capitol and Daly 

 peaks, sweep around to the north of this station and turn to the south- 

 ward, crossing Aspen Creek, as described on the preceding page. Here 

 the entire series is exposed, there only the Upper Cretaceous and bits 

 of the Paleozoic rocks in the bed of the creek. The lines of outcrop can 

 be traced between the two points. The older rocks gradually disappear 

 as the granite begins to fault up past the broken edges. (See colored 

 map). This may be regarded as the farthest northern extension of the 

 great fault-fold previously mentioned. This fold being a most compli- 

 cated and interesting piece of dynamics, calls for a separate analysis, 

 which I give farther on. 



On the 3d we descended Aspen Creek to the main creek and continued 

 the examination of the Cretaceous section. The black shales, the upper 

 part of which are exposed in the bluff on the north side of Aspen Creek, 

 occupy the valley from the base of the bluff to the base of Treasury 



