106 GEOLOGY OF THE DENVER BASIN. 



The upper member of the Trias, within the limits of the area under 

 consideration, rapidly diminishes from its normal thickness of aboul 600 

 feel a1 either end to complete disappearance a1 Gregory Canyon, near the 

 crntcr of the affected region. Measurements at two points along the line 

 of gradual disappearance — -one just north of Boulder Creek, between it 

 and the wagon road leading to Sunshine, the other a few hundred yards 

 south of Gregory Canyon, near the northern termination of its southern 

 outcrop — gave a thickness, respectively, of 350 and , r >0 feet. It is the lower 

 beds of the member that are the last to disappear, the limestones near its 

 base appearing at several points after the formation has begun to thin out. 

 Destruction of its beds has apparently been carried to its very base, though 

 not beyond. 



The jura. — This formation, for the greater portion of its outcrop within 

 the affected area, shows a marked decrease from the normal thickness, I nit 

 nowhere disappears entirely. The decrease first becomes apparent, at the 

 north, in the vicinity of the Sunshine road; at the south, in a gulch about 

 three-fourths of a mile south of Gregory Canyon. The diminution almost 

 wholly occurs near the limits of the area of unconformity, a considerable 

 distance along the center showing a constant thickness of about 50 feet. 

 The evidence of the strata in explanation of the thinning is in favor of non- 

 deposition of the lower beds, the characteristic conglomerate and mottled 

 sandstone near the summit of the formation appearing at several points 

 along the trend of the strata. 



The Dakota. — This thins very perceptil >1 v between Polecat Canyon and 

 the gulch next north, having within the intervening half mile decreased in 

 thickness from nearly normal to about 100 feet, with a, complete disappear- 

 ance of the hogback features. At Gregory Canyon the decrease is still 

 greater, the actual exposure of Dakota here being only about I feet, but 

 with a covered thickness of probably :'><» feet more. Immediately north of 

 Boulder Creek it is still at its minimum, but at Sunshine Gulch it is rapidly 

 regaining its width — at this point being approximately 150 feet — while a 

 half mile farther north it has again reached its normal thickness. ( >ver the 

 area of minimum thickness it is quite impossible to determine whether it is 

 the upper, middle, or lower portion of the formation which remains, but 



