WALCOTT. 



SUMMARY TEXAS. 



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Wyoming , 



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Colorado 



On the atlas sheet issued by the Wheeler Survey in 1874 the Silurian 

 (which also included the Tonto formation) is represented as extending 

 from the head of the Grand Canon along the entire course of the canon 

 to its mouth, and thence southward along the western margin of the 

 Colorado plateau to Music Mountain, where the outcrop broadens out 

 and extends southwest along the south- 

 western margin of the plateau. The most 

 southeastern outcrop is on the Rio Verde 

 River near the lower end of Williamtowu 

 Valley. 



In all the sections on the line of the 

 Grand Canon and the western and south- 

 western margins of the Colorado Plateau 

 the Tonto sandstone rests uncomforinably 

 upon either Algonkian or Archean rocks. 

 In its greatest development the formation 

 attains a thickness of 1,000 feet and rep- 

 resents the Upper Cambrian zone, and 

 possibly in the lower portion, the Middle 

 Cambrian. 



RESUME. 



The sediments of this province are those 

 deposited as the sea advanced slowly upon 

 the laud and distributed the debris worn 

 from it or brought in by tributary streams 

 as layers of sand a!ong the coast line and 

 in an adjoining shallow sea. From Ari- 

 zona to Texas, to Missouri, the Black 

 Hills, and the eastern margin of the Rocky 

 Mountains, and all along the northern line 

 in Minnesota, Wisconsin, Michigan, Can- 

 ada, and the Adirondacks of Xew York 

 the record is the same — sandstones resting 

 upon pre-Cambrian rocks and carrying 

 essentially the same fauna. In some re- 

 gions the depth of water increased more 

 rapidly and calcareous deposits accumu- 

 lated upon the sand, as in Arizona, Texas, 

 the Black Hills, and in some localities 

 along the eastern base of the Rocky 

 Mountains. That the sea was shallow, wide spread, and advancing 

 along a great extent of coast line is shown by the sediments and the 

 presence of a similar fauna in Xew York, the Upper Mississippi Valley, 

 the Black Hills, the eastern front of the Rocky Mountains, and far to 

 the south in Texas and Arizona. All over this broad area the rela- 



Western. Wisconsin 



%% 



CzntraZ Wisconsin. 



astern. Wisconsin, 



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