154 W. O. CROSBY — ARCHEAN-CAMBRIAN CONTACT IN COLORADO 



east 15 degrees. Thegranite that first appears in the bottom of the canyon 

 evidently forms a broadly rounded erosion boss nearly 100 feet in its north- 

 south diameter, with the strata curving smoothly over it. A similar but 

 somewhat smaller swell of the strata immediately south of this undoubt- 

 edly indicates another boss not quite uncovered. The granite shows here 



only on the west side of the 

 road, so that the east- west pro- 

 file of the boss must be rather 

 al)rupt (figure 30). The granite 

 is bright red, of fine to medium 

 texture, and, what is of special 

 interest, the outcrop or boss 

 shows more or less across its 

 entire breadth, but particularly 

 at the southern edge, a distinct 



-East-xcest Section of the Contact in Williams ^^^^^^^^^^^^ ^f granite frag- 

 Canyon. " ° " ' 



ments and pebbles (mostl}^ well 

 rounded) between the solid granite and the white sandstone. The sand- 

 stone forms the matrix or paste of the conglomerate, and the fragments 

 of granite extend upward several inches in the sandstone. In fact, where 

 most distinctl}^ shown, the conglomerate appears to be about 2 feet thick, 

 and the largest granite pebbles are perhaps 6 inches in diameter. They 

 are very closely packed, and the conglomerate grades downward into 

 the weathered joint structure of the granite, so that there is probabl}^ not 

 more than 1 foot of true conglomerate. 



North of this swell of granite the contact is exposed continuously for 

 over 200 feet. It is broadly undulating, with often a trace and at one 



M. 



Figure 30.- 



i inch = lOf eet. 



Figure 31. — Irrcfiulnr Erosion Contact of the Sandstone and gncissoid Granite in Williams Canyon. 



point fully a foot of granite conglomerate separating the granite and 

 sandstone. The surface of the granite now descends below the level of the 

 road for al)out one hundred feet, and then with broad undulations the 



