nofONS.] GLENWOOD SPRINGS TO LEADVILLE. 415 



After leaving Glenwood the strata gradually rise, and beyond the 

 tunnels the Arehean rocks are suddenly brought up, apparently by a 

 fault. They consist of red and gray granites, inclosing masses of 

 gray gneiss and dark amphibolite, and traversed by irregular veins of 

 pegmatite. These rise, midway in the canyon, 500 to 1,000 feet above 

 the riverbed, and gradually descend to the eastward. Their uncon- 

 formable contact with the overlying Paleozoic rocks can be distinctly 

 traced for a long distance on cither side of the river. At the east end 

 of the canyon the harder Lower Paleozoic rocks (Cambrian, Silurian, 

 and Lower Carboniferous) gradually sink below the river level, and the 

 valley widens out in the overlying gypsiferous beds. 



At Dotsero the road leaves the valley of the Grand, which bends to 

 the north, to follow that of its main tributary, Eagle river," 3 which 

 conies in from the east. Above the junction the hills on either side 

 are capped by Triassic beds, forming the southeast fringe of the White 

 River plateau. On the summit of those to the north is a basalt vent of 

 extremely recent age, whose lava has run down the southwest face of 

 the cliff and flowed for a considerable distance along the valley bottom, 

 crowding the stream to its southern edge. The track passes for a half 

 mile close to this remarkable coulee, and the singularly rough, ropy, 

 and scoriaceous nature of its surface can be readily seen from the train. 

 The absence of any evidence of erosion, and its relation to the river 

 (Travels unite to prove it the most recent known lava tlow in Colo- 



rado. 64 



Before reaching Gypsum station the road enters the considerable 

 east and west valley of the same name, so called from the abundant 

 beds of gypsum in the soft shales and limestones of Carboniferous age, 

 out of which it has been eroded. It is about 15 miles (24 km.) in length 

 and occupies a gentle anticline in the contorted and folded gypsiferous 

 beds, which form part of a general synclinorium included between the 

 Park and Sawateh ranges on the east, and the White River plateau 

 and Elk mountains on the west. These beds are peculiar to this region, 

 the same horizon in other parts of Colorado being represented mainly 

 by coarse grits with a very subordinate development of shales and 

 limestones. 



The valley has at one time been occupied by a fresh-water lake, pre- 

 sumably of Pliocene age. in Which were deposited a series of soft pink 

 marls, remnants of which can be seen on the north side of the river. 

 Still more recent deposits of waterworn gravels, probably brought 

 down by the tloods which occurred at the close of the Glacial period, 

 are exposed in the railroad cuts. 



At the eastern end of the valley the gypsiferous beds sink down 

 below the valley level and give place to sharply upturned Bed Beds 

 (Trias). The valley now bends to the northeast, and then takes a sharp 



