42G GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



At Parkdale is a considerable open valley, in which are nearly hori- 

 zontal Cretaceous beds lying- unconformably upon Silurian limestones. 

 These were deposited in an ancient bay, which connected with the 

 ocean to the eastward, both to the north and south of the Arehean mass 

 through which the railroad now passes. 



For nearly 10 miles the road now follows a dec]) canyon, known as 

 the Royal Gorge, whose walls rise almost perpendicularly above the 

 track two or three thousand feet. This is probably the most imposing 

 canyon gorge that is traversed by railroads in Colorado. The rocks are 

 largely dark gneiss and amphibolite, cut by eruptive granite, pegma- 

 tite veins, and narrow diabase dikes, and show distinct structure lines 

 which stand almost perpendicular, dust as the river emerges from the 

 canyon it is joined by Grape creek, a rapid stream which comes down 

 another narrow, winding gorge from the northwest portion of Wet 

 Mountain valley. Just east of tho mouth of Grape creek, and on the 

 south side of the river, is a low ridge of Silurian sandstones and lime 

 stones resting directly upon the Arehean. At the northern end of this 

 ridge are hot Springs, where are bath houses and a hotel adjoining. 

 The thermal waters issue from the Silurian limestones. 



From the Arehean gateway of the Royal (Jorge the road passes for a 

 mile across an interior monoclinal valley between the foothills and 

 a hogback ridge, formed by the hard sandstones of the Dakota 

 Cretaceous. This monoclinal valley stretches ten miles due north, 

 parallel to the eastern slope of the mountains. It is bounded on the 

 east by a very steep wall, formed by the edges of the Dakota sand- 

 stones, and slopes gently up to the west against the Hanks of the 

 Arehean. These slopes are made up of Silurian and Lower Carbon- 

 iferous sandstones and limestones resting unconiformably upon the 

 Arehean, and in turn uncomformably overlaid by limestone breccia and 

 soft arkose sandstones, whose age has not yet been definitely deter- 

 mined, but is either Carboniferous or Trias. Out of the softer rocks 

 the bottom of the valley has been eroded. Above them rest the Jura 

 Dakota sandstones, again unconformable, though where crossed by the 

 railroad there is no marked discrepancy in the angle of dip. Bast of 

 the Hogback ridge are quarries in Niobrara limestone, worked by con- 

 victs from the State penitentiary at Canyon City. 



