

368 THE CAMBRIAN. Tbull.81. 



and protected shores of the seaward slope of the western ridge, where 

 the outer or eastern ridge was absent, as in Massachusetts and New- 

 foundland. The sediments that accumulated to the eastward of the 

 New Brunswick sea form the supposed Cambrian shales and slates of 

 Nova Scotia. The inner ridges of Maine, New Hampshire, and Massa- 

 chusetts bounded long, narrow seas, in which the Cambrian faunas are 

 not known to have penetrated. The Lower Cambrian fauna probably 

 passed from the Atlantic along the ancient Labrador shore into the 

 interior Appalachian sea. A few types of the Middle Cambrian fauna 

 followed, and then the passage appears to have been closed, as none of 

 the Upper Cambrian types of the Atlantic fauna have been found in 

 the deposits of the interior seas. 



The various views, both theoretical and hypothetical, relating to the 

 pre Cambrian continent are embodied on the map, PL in, and the sec- 

 tion at the base of PL n. 



DESCRIPTION OF PLATE III. 



Hypothetical map of the North American Continent at the beginning of and during Lower 



Cambrian time. 



This map is based'upou the sections shown on PL n, and the theoretic section at 

 the base. The position of the sections on the two maps is indicated by a circle with 

 a corresponding number on each map. 



The shaded portions indicate the relative areas that are supposed lo have been 

 above the ocean during later Algonkian and Lower Cambrian time. A comparison. 

 with PI. II shows that in the Rocky Mountain and Appalachian areas there was an 

 enormous accumulation of sediment prior to the Upper Cambrian; while over the 

 broad interior there is only the Upper Cambrian which corresponds to the upper por- 

 tion of the great sections of the Rocky Mountain and Appalachian areas. In the 

 region of the Atlantic Coast Province the continental movement seems to have been 

 less, as the sediments of the Lower and Middle Cambrian are much thinner than 

 those of the corresponding horizon in the Appalachian and Rocky Mountain areas. 

 The conclusion is that the shaded portions and perhaps other unknown areas formed 

 the land during late Algonkian and early Cambrian time, and that about the close 

 of the Middle Cambrian or the beginning of Upper Cambrian time the continental 

 surface was largely depressed beneath the waters of the ocean and the Upper Cam- 

 brian sediment accumulated over the entire area where Cambrian rocks are now 

 known to occur. (See distribution on PL I.) 



On PI. in, A., Archean ; K., Keweenawan ; B. H., Black Hills of Dakota; T., Llano 

 area of Texas ; M., Ozark uplift of Missouri ; C, Grand CaFion area of Arizona. The 

 area marked X, X, X indicates an hypothetical land area of the existence of which 

 we have not at present any absolute proof, as it is now covered by sediments of later 

 age than the Cambrian. * 



The present shore outlines of the continent are indicated by the dotted line, and the 

 outlines of the continental plateau on the division between the deep sea and the con- 

 tinent by a continuous line. 



The descriptive notes of the Cambrian rocks of the various provinces 

 and the resume following each establishes the fact .that in Cambrian 

 time there were geographic areas in which different types of sedimeuta- 



