THE CAMBRIAN PERIOD. 221 



areas which remained above the sea up to the end of the Middle Cam- 

 brian. In Mexico and Central America, the blank area represents 

 lack of knowledge. Since the distribution of the Lower and Middle 

 Cambrian series is much the same, the black areas of the map may 

 be taken to represent the surface distribution of the Lower Cambrian 

 without serious error, if the areas in Arizona, Oklahoma, and Texas 

 be excluded. On account of later erosion, the Lower and Middle 

 Cambrian strata are probably not now present over the whole of the 

 area on which they were originally deposited. 



So far as these formations contain marine fossils, it is safely inferred 

 that the areas where they now occur were occupied by the sea at some 

 time during the early and middle portions of the Cambrian period. 



As the map shows, the Lower Cambrian is known on the coast of 

 Labrador, in Newfoundland, at various points between these localities 

 and Maine, at several places in New England, at many points in the 

 Hudson-Champlain valley, and at various points in the Appalachian 

 mountains between the Hudson and Alabama, but is generally absent 

 from the area east of these mountains. From the distribution of the 

 Lower Cambrian series in the east, and from its fossils, it is inferred that 

 a great sound existed in early Cambrian time near the southeastern 

 border of the continent, extending from Labrador to Alabama. 1 This 

 sound is believed to have been shut off from the Atlantic by land which 

 occupied a large part of the site of the present Piedmont plateau, 

 but whose eastern limit is unknown. 



Lower Cambrian formations, with fossils of marine organisms, are 

 also known in Nevada, L T tah, western Montana, Idaho, and in some 

 parts of British Columbia, from which it is likewise inferred that the sea 

 occupied portions of western North America during the same epoch. 



The absence of Lower Cambrian strata from other regions is hardly 

 less significant than their presence in those mentioned. They are 

 believed to be wanting in most of Montana, in Wyoming, Colorado 

 and Arizona, and in the territory east of these states nearly to the 

 Appalachian mountains. 2 Not only are they absent now, but they 

 appear never to have been present, and this leads to the inference that 



1 TJlrich and Schuchert, Bull. 52 (Paleontology 6) New York State Museum, Report 

 of the State Paleontologist, 1901, pp. 635-6. 



2 Comstock (First Ann. Rept. Geol. Surv. of Texas, 1889, pp. 285-292) thinks 

 Lower Cambrian occurs in Texas. 



