94 GEOLOGY OF THE DENVER BASIN. 



sediments into the fold traced in Profile III, IM. X, the rise of the arch in this 

 case being approximately 1,000 feet. The figure shows the character of 

 the fold on the line of profile given to have been that of a long, gentle 

 slope from the confines well toward the center, where, on further yielding 

 to the compressive forces, a clearly defined median ridge was produced. 

 Erosion naturally went on in a more or less irregular manner, but the 

 general position of the hill and its component strata relative to erosive 

 forces was apparently such as to cause the disappearance from the top of 

 the Jura, over those parts of the slopes of gentle inclination, of only the 

 most insignificant amounts of material, while over the central or sharper 

 portion of the fold the probable effect was the complete removal of the 

 beds of the Jura and Upper Trias, together with a partial removal of those 

 of the Lower 'Trias, from the crown of the arch, and of the material from 

 the adjoining Hanks down to the gently sloping line of union shown 

 between these formations and the Dakota lying across their edges. 

 Whether erosion reached an extent sufficient to permit the deposition of 

 the Dakota and the lower part of the Benton entirely across this rise is 

 doubtful, but from the rate of disappearance of the Dakota from below, it 

 is probable that neither this formation nor the lower half of the Benton 

 was here laid down. 



The evidence for the conclusions given in the preceding statement is 

 clearly brought out in the strikes (ancient dips) and surface relations of 

 the formations to one another, notably, in the divergence in strike and the 

 truncation of the edges of the Jura by the Dakota on the southern side of 

 the ,»a]) (Profile 111, l'l. X); in the thinning of the Dakota in such a man- 

 ner as to eventually leave the fire-clay in its upper half in contact with 

 the older sediments at the two points where the formation appears to end, 

 in the south bank of Clear Creek and the north one of Gold Run; and 

 in the ready reproduction by graphic methods of the structural conditions 

 observed in the field and the natural sequence of events based thereon. 



Sedimentation of the Dakota, Benton, and Niobrara continued unin- 

 terruptedly to the close of the latter time, subsidence probably keeping 

 pace. With this the third period of development ended. 



Fourth period. — The fourth period embraces the time during which the 

 great elevation shown in Profile IV, PI. X, was created, and in which 



