364 THE CAMBRIAN. [bull. 81. 



some exceptions, as, for instance, the deeper water limestone series of 

 central Nevada, British Columbia, and western Vermont, and per- 

 haps the black shales of the Atlantic Coast Province. The assembled 

 evidence sustains the view that at the beginning of Lower Cambrian 

 time the area of the great Interior Province formed part of a continent, 

 to the eastward and westward of which long ridges of pre-Cambrian 

 rock separated interior seas aud straits from the continental area and 

 protected their contained life and sedimeuts from the ravages of the 

 open ocean. As the continent was slowly depressed and the waters 

 advanced upon the land the sediments now forming the rocks of the 

 Lower and Middle Cambrian series were accumulated in the various 

 interior bodies of water to the eastward and westward of the main 

 land area and between it aud the outlying ridges. What the contour 

 of the south and southeastern side of the continent was and to 

 what extent the sea advanced upon it from the south during this 

 time is unknown and may never be known, as only the formations 

 that were deposited around the pre-Cambrian islands of Texas and 

 Missouri are now accessible for study. From the evidence afforded 

 by these two localities and that along the eastern front of the Rocky 

 Mountains, and the exposures of Cambrian strata in Wisconsin, Can- 

 ada, etc., it is very probable that the main portion of the continent 

 north of the thirtieth and south of the fiftieth meridian did not dis- 

 appear beneath the advancing sea until near the beginning of Upper 

 Cambrian time. The unconformable position of the Upper Cambrian 

 rocks of the Interior Continental Province upon the subjacent Algon- 

 kian and Archean rocks sustains this conclusion. 



As the sea was transgressing over the surface of the continent on 

 its way northward across the broad interior in late Middle or early 

 Upper Cambrian time it was also working along the base of the border 

 ridges and depositing the sediments derived from them conformably 

 upon those deposited, while the main mass of the continent was above 

 the water. That these deposits were practically contemporaneous with 

 those of the Interior Province is proved by the presence of the same 

 types of animal life, and to a considerable extent of identical species. 



Toward the close of Cambrian time a large portion of the continent 

 had disappeared beneath the surface of the sea, and the great lime- 

 stone-forming period of the Silurian (Ordovician) began. In some 

 areas, as about the Adirondack Mountains of New York, argillaceous 

 and arenaceous sediments were derived from the adjoining coast line, 

 but as a whole mechanical sediments are absent. 1 



'In speaking of the conditions of sedimentation Messrs. Campbell and Ruffher state that "Changes 

 such as these occurred, during a series of geological ases of unknown length, in a great inland sea 

 which was once connected with what is now the Gulf of Mexico, on the south, limited probably by 

 the highlands of Canada on the northeast, having the Archean ledges of the Blue Ridge for iu 

 southeastern border, and in all probability separated, in part at least, from the Pacific Ocean by the 

 Rocky Mountain Range. This extensive sea. with Archean rocks lor its bottom and shores, was the 

 receptacle of the various materials that now constitute the surface rocks and soils of the Mississippi 

 Valley." (A physical survey extending from Atlanta, Georgia, across Alabama and Mississippi to 

 the Mississippi River, along the line of the Georgia Pacific Railway. New York, 1883, pp. 9, 10.) 



