WALCOTT.j SYNOPSIS. 367 



Section 19. Section of the Upper Cambrian sandstones of eastern Wisconsin. 



Section 20. Section of southern central Wisconsin, showing the unconformity 

 between the Upper Cambrian and the subjacent Algonkian and Archean rocks. Sec- 

 tions 18, 19, and 20 are all of Upper Cambrian age, and pass conformably above into 

 the superjacent Lower Silurian (Ordovician) or magnesian limestones. 



Section 21, Ozark Mountains, southeastern Missouri. — The relations of the Cam- 

 brian and the Archean are the same as those in the Black Hills section (section 23)- 



Eastern or Adirondack snbprovince: 



Section 22. The eastern and northern slopes of the Adirondack Mountains of New 

 York. — In this section the Potsdam sandstone of the Upper Cambrian rests uncon- 

 formably upon and against the Archean and Algonkian rocks. 



Western Interior Continental or Rocky Mountain sub-province: 



Section 23. Black Hills, Dakota. — The Upper Cambrian rests unconformably upon 

 the Archean. 



. Section 24. Eastern section of Big Horn Mountains of Wyoming. — It is essentially 

 the same as that of the Black Hills (section 23). 



Section 25. Southern Montana. —This section is very much like that of Wyom- 

 ing and of the Black Hills. 



Section 26. Central Colorado. — A section representing the sandstones that lie be- 

 tween the subjacent unconformable Archean or Algonkian rocks and the superjacent 

 Lower Silurian (Ordovician). 



Southwestern Interior Continental sub-province: 



Section 27. Grand Canon of the Colorado, northern Arizona. — In this section the 

 Cambrian strata are uncomformably superjacent to rocks of Algonkian age. 



Section 28. Llano County, Texas. — This section is similar to that of the Grand 

 Canon in having an unconformity between the Algonkian and the Cambrian and in 

 representing nearly the same geologic horizon. 



Theoretic sections at close of Cambrian time. — The data for these sections are ob- 

 tained from the associated map. The enormous accumulation of sediment in the 

 Rocky Mountain Basin on the west and the Appalachian Basin or trough on the 

 east are shown, as well as the thin mantle of sediment over the great interior. It 

 serves to illustrate the sedimentation indicated by the vertical columns on the map. 



Our knowledge of the sediments of the eastern and western sides of 

 the pre-Canibrian continent is considerable, but of that deposited along 

 the southward-facing front we know nothing. From the fact, how- 

 ever, that the same species of fossils occur in the Lower Cambrian 

 fauna of Labrador, Massachusetts, New York, Vermont, probably 

 Tennessee, Nevada, and British Columbia, I think we may hypotheti- 

 cally assume the continuance of the Lower Cambrian beneath the de- 

 posits of the Gulf States and westward through Texas, New Mexico, 

 and Arizona. There is no known line of Lower and Middle Cambrian 

 sedimentation across the continent to the north of this which indicates 

 that the fauna might have been distributed along a more northern 

 shore. 



The pre-Cambrian ridges, or protaxis of the present ranges of the 

 northeastern side of the continent, have been outlined by Prof. J. D. 

 Dana from the known exposures of pre-Cambrian rocks. 1 The Para- 

 doxides fauna lived in the depression between two of the eastern ridges 

 of the Atlantic Province in the New Brunswick area, and in the bays 



'Areas of continental progress in North America and the influence of the condition of these areas oxj 

 the work carried forward within thein. Bull. Geol. Soc. Ani., vol. 1, 1890, pp. 36-3S* 



