COMPARISON WITH OTHER REGIONS 



157 



JJ,^ 



3 c 



y? CD 



IT o 



3S 



"COtOftADO 



a view of the contact taken on Box Elder creek, on the east side of the 

 Hills (plate 18). This suffices at least to show that irregularities, if not 

 entirely Avanting here, occur only on a very minor scale. 



I have searched the literature of Rocky 

 Mountain geology in vain for any detailed 

 description of this important contact, 

 which may without exaggeration be de- 

 scribed as at once the most widespread, 

 strongly accentuated, and deeply signifi- 

 cant structure plane of all the trans-Mis- 

 sissippian region. It has been frequently 

 referred to in general terms and figured 

 diagrammatically, the figures being gen- 

 erall}'-, like Hayden's, consistent with a 

 highly plane or featureless Archean floor 

 of the Cambrian sea. Several of the folios 

 of the United States Geological Survey for 

 Colorado, Montana, etcetera, show this 

 contact in both maps and sections, but 

 the texts and illustrations alike are want- 

 ing in details. 



We are indebted to Walcott* for the 

 best general summary of our knowledge 

 of the Archean-Cambrian (not necessarily 

 Potsdam) contact in North America. In 

 the section on the surface of the pre- 

 Cambrian land the original and quoted 

 descriptions, wherever topographically ex- 

 plicit, indicate in general that erosion in 

 each region had nearly or quite completed 

 its task of reducing the land to a perfect 

 baselevel before the deposition of the Cam- 

 brian sediments began. 



In describing the Grand Canyon section 

 of northern Arizona (figure 33), Walcott 

 says : 



"A baselevel of erosion appears to have been 

 reached before the Cambrian rocks (Tonto group) 

 exposed on the line of the canyon section were 

 deposited. Both the Archean and Algonliian rocks were eroded nearly to a hori- 

 zontal plane prior to the deposition of the Upper Cambrian sandstone. Here and 



U^JffA/* vall£Y. 



*Tlie North American Continent during Cambrian time, 12th Ann. Rep. U. S. Geol. Survey, pp. 

 523-5C8. 



XXIV— Bull. Geol. Soc. Am., Vol 10, 1898 



