Ill NV. C). (MJOSP.Y .Vi:i'IIICAX-("AM mn AN CONTACT IN COI.OKAIM) 



niaruiii. owing its lohatc (.'harju'tor wholly to the topo^raiiliir rehitions, 

 since the strata natarally extend farther np the slope ot" tlie mountains 

 on tlie ridges than in the valleys. The third or eastern houndary of the 

 end)aynient, which may be regarded as a triai\gle extending from Camp 

 creek on the north to Bearereek on the south, witii its apex in Ute pass, 

 is well nuirked l>y the outcrop or hogback oi' the vertical Dakota sand- 

 stone. The three sides of the embayment are thus (southwest) a pro- 

 found obliquely transverse fault,(east) a precipitous monocline or possible 

 k>ngitudinal fault, and (northwest) the intersection of the basal beds or 

 Cambrian sandstone by a highh" irregular erosion surface, and it is with 

 this lobate northwest margin that we are now specially concerned. 



North of the embayment, along the base of the Frtint range, tlie pre- 

 vailing strike is north-south and the dip easterly, the lower terranes 

 usually sloping gently away from the granite, while the Red beds (Tri- 

 assic) and higher formations plunge steeply down beneath the plains. 

 The embayment has its origin in the Ute fault, a drop ahuig an obliijue 

 fracture equal at least to the combined thickness of the entire Paleozoic 

 and Mesozoic terranes. This has bent the beds around to a northeast- 

 southwest strike and southeast dip, and, as before, the di[> is moderate 

 (10 to 30 degrees) below the base of the lied beds and steep above. Along 

 the southwest margin, however, each formation is in turn abruptly up- 

 turned to a steep northeast dip by the drag of the Ute fault. In fact, at 

 some points along this line the beds are overturned to a high southwest 

 dip, as we should naturally expect, since the great tault belongs in gen- 

 eral to the reversed or overthrust type, with a haile to the upthrow side 

 (southwest) of 5 to 45 degrees. 



Archean-Cambrian Contact 



Of the two granite or Archean borders of the embayment, one (south- 

 west) is secondary and abrupt, marking a profound dis[)lacement, while 

 the other (northwest) is original — the gently sloping floor on which tlie 

 sediments were deposited. The granite is the coarse and (^superficially) 

 red biotite granite so characteristic of the region and remarkable for its 

 extensive surface disintegration. It is cut occasionally by dikes or irreg- 

 ular masses of a finer granite, in which the biotite is less prominent, and 

 it exhibits at some points, as in Williams canyon, a marked gneissoid 

 structure, with a relativel}'' basic or even dioritic fades. 



The general character of -the Cambrian sandstone, the basal member 

 of the sedimentary series, has been indicated. According to Peale,* it 

 embraces in Glen Eyrie, from below upward, coarse grayish white .sand- 



* Ann. Rep. U. S. Gepl. and Geog. Surv. of the Territories, 1873, p. 201. 



