464 HISTORICAL GEOLOGY. 



Cambrian, and in Murchison's expressing his satisfaction that geologists and 

 paleontologists everywhere, in America as well as in Europe, had already- 

 adopted, through the use of his publications, his subdivisions and terms. 

 Later, after collections of Cambrian or Primordial fossils had been much 

 enlarged through new discoveries, the names Cambrian and Lower Silurian 

 became accepted for successive divisions of the Paleozoic series. 



The term Cambrian is derived from the old name of Wales, and Silurian 

 from the tribe of Silures, which inhabited southeastern Wales and Mon- 

 mouth, England. 



For a more detailed history of the terms Cambrian and Silurian, see the Am. Jour. 

 iSc, xxxix., 1890 ; also Murchison's Life by A. Geikie, 1875. 



AMERICAN. 



Subdivisions. 



3. Potsdam period. Reports Neiu York Geologists, 1838, 1842. Upper 

 Cambrian, Walcott. Later Cambrian. 



2. Acadian period, Dawson, Acad. Geol, 1868. Middle Cambrian, or 

 Paradoxides zone, Walcott, 1887. ISTamed Acadian from the locality at 

 St. John, ISTew Brunswick. 



1. Georgian period, 1886; Lower Cambrian or Olenellus zone, 1887, 

 C. D. Walcott, Bull. U. S. G. S. Keweenawian, T. B. Brooks, Am. Jour. Sc, 

 xi. 206, 1876; Keweenawan, Chamberlin, 1883; Irving, 1887; Keweenian, 

 A. Winchell, 1886. 



ROCKS — KINDS AND DISTRIBUTION, 



General Distribution. — The Cambrian rocks rest upon the upturned 

 Archaean terranes, and usually outcrop along the borders of Archaean areas. 

 In eastern North America they occur, adjoining the Archaean nucleus, on 

 one or both sides of the Appalachian protaxis, from Canada to Alabama, and 

 occupy parts of some, if not all, of the channels or troughs of Archaean con- 

 fines from the Adirondacks to the eastern limits of Newfoundland. They 

 are in part beach-made and wind-made sandstones, or offshore limestones, or 

 slates or schists that originally were mud beds ; and the layers often bear 

 ripple-marks, shrinkage cracks, worm-burrows, and, in some places, tracks 

 of animals. 



Similar relations to the Archaean exist at various localities of the Lower 

 Cambrian over the continent, to the far west. They are found about Ar- 

 chaean outcrops in Texas and South Dakota, and along the Rocky Mountain 

 protaxis in British America and the United States, and also farther west in 

 Nevada ; and occasionally they are reached, over the Pacific slope, by the 

 canon cuts of rivers thousands of feet in depth, as in that of the Colorado. 



The accompanying sketch of a portion of the " Pictured Rocks " in the 

 Lake Superior sandstone, near Carp River, Michigan, illustrates the usual 



