30i Schuchert — Cambrian of the Grand Canyon of Arizona. 



The Cambrian sea " transgressed a surface which strongly 

 resembled 1he present surface of the great Laurentian peneplain 

 of Canada with its broad areas of crystalline rocks in which are 

 inset occasional blocks of Paleozoic strata, and above which stand 

 occasional monadnocks of quartzite. . . . 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. . . . These monad- 



Fig. 4. i 



Fig. 4. General view of Grand Canyon from Tonto Platform east of Cope 

 Butre. looking west. I, Lower Snpai ; II, Redwall limestone ; III, Muav 

 formation; IV, Bright Angel shale; V, Tapeats sandstone; VI, Vishnu series. 



nocks of the Cambrian plain may be compared with the Baraboo 

 ridges of Huronian quartzite [in Wisconsin] which by virtue of 

 their homogeneity and hardness still stand as prominences which 

 have weathered repeated cycles of erosion." 



Subdivisions and Character of the Middle Cambrian 

 Series. — According to Doctor Noble, 2 the basal member 

 of the Cambrian series is the Tapeats sandstone with a 



2 L. F. Noble, Geol. Hist., Bright Angel Quadrangle, U. S. Geol. Survey, 

 1914. 



