The Shinumo Area. 527 



ite, resisting the forces of erosion in that ancient plain by 

 virtue of the terrific hardness which causes the same strata 

 to-day to wall in the deep box-canyon of the Shinumo. These 

 monadnocks of the Cambrian plain may be compared with the 

 Baraboo ridges of Huronian quartzite which by virtue of their 

 homogeneity and hardness still stand as prominences which 

 have weathered repeated cycles of erosion. It is probable 

 that this cycle of erosion was not finally completed until well 

 along in lower Cambrian time. 



The next' event is the incoming of the Tonto sea. Although 

 this chapter belongs to the Paleozoic history recorded in the 

 horizontal strata of the walls of the Grand Canyon, that part 

 which is involved with the distribution of the Unkar strata 

 upon the pre-Tonto surface may properly be anticipated here. 



It is not unlikely that this invading sea transgressed a sur- 

 face which strongly resembled the present surface of the 

 great Laurentian peneplain of Canada with its broad areas of 

 crystalline rocks in which are inset occasional blocks of Paleo- 

 zoic strata, and above which stand occasional monadnocks of 

 quartzite. The story of the invading sea is written in the 

 Tonto Sandstone, locked up in which is a record of marine 

 transgression which on account of the vertical sections and 

 absence of soil is clear beyond belief. "When the Tonto sea 

 came in over the surface of the ancient peneplain the monad- 

 nocks stood out as islands which were gradually overwhelmed 

 and buried in the sands of the deepening sea. The long ridge 

 of the quartzites of the middle Unkar in the Shinumo area has 

 already been described. It stood out as a long narrow rocky 

 island whose longer axis extended for an unknown distance 

 northwestward along the strike of the strata. The long south- 

 west face of this island was undercut by the marine planation 

 to form a steep cliff. Every detail of the face of this old sea 

 cliff is preserved in the Tonto sandstone in the cross-sections 

 cut by the present canyons across its face : at its base, huge 

 angular blocks of Unkar quartzite as large as houses are pre- 

 served in the Tonto sandstone in the exact position where they 

 fell and lodged ; farther out from the base are masses of large 

 bowlders, worn and rounded by the pounding of the waves ; 

 these become smaller and smaller and finally run out into 

 lenses of fine pebbly conglomerate, representing the shingle of 

 the ancient beach, dragged out by the undertow. ~No more 

 striking example of a fossil sea cliff can be imagined. 



By far the most impressive feature of this wonderful coun- 

 try is to traveller and geologist alike the mile-deep pathway 

 of the Colorado River of the West across the great plateaus. 

 The stupendous and glaring record of erosion revealed to us 



