THE CAMBRIAN PERIOD 59 



edges only are now exposed, the surface distribution as indicated 

 on the map fails to give any adequate idea of the former or even 

 present real extent of strata of this age. Thus Cambrian strata 

 are definitely known to have been almost completely removed 

 from several thousand square miles of the northern New York 

 region, and Cambrian rocks have certainly been similarly removed 

 from many other regions. Again, the distribution of the outcrops, 

 together with many deep well sections (Fig. 27), make it certain 

 that Cambrian strata concealed under nearly horizontal later strata 

 spread across much, if not all, of the Mississippi Valley from the 

 Rockies to the Appalachians, while in the Appalachian Mountains 

 Cambrian rocks are really much more extensive than the mere 

 outcropping edges of the upturned strata. There is no reason, 

 however, to think that the vast area of pre-Cambrian rock around 

 Hudson Bay, the Atlantic Coast from New Jersey southward, the 

 Pacific Coast including the Coast Range, Sierra Nevada, and 

 Cascade Mountain areas, were ever covered by the Cambrian 

 sea. 



The difference in the distribution of the Lower and Upper 

 Cambrian strata is a prime consideration. Thus the Lower Cam- 

 brian is entirely absent from the whole Mississippi Valley except 

 probably its southern border. Otherwise the same general areas 

 are occupied by both Lower and Upper Cambrian strata. 



Character of the Rocks. — Cambrian rocks consist very largely 

 of shallow water sediments such as conglomerates, sandstones 

 (Fig. 28), and shales, with well-preserved ripple marks very 

 common. Deeper or clearer water deposits such as limestone, 

 are, however, important in the Appalachians, Vermont, Nevada, 

 and British Columbia. When these sediments were deposited 

 in the Cambrian sea they were like ordinary gravels, sands, marls, 

 and limy oozes now forming in the ocean, especially over the 

 continental shelf areas and their borders. Since their deposition 

 they have been changed into the corresponding harder rocks such 

 as conglomerates, sandstones, shales, and limestones, or, in some 

 cases as in New England, metamorphosed into quartzites, schists 

 or slates, and crystalline limestones (marbles). In many regions 

 the Cambrian strata have been highly folded and faulted. 



Thickness of the Cambrian and Igneous Rocks. — The thick- 

 ness of Cambrian strata in North America varies from less than 

 1,000 feet to a maximum of over 10,000 feet. North American 



