JAMES DWIGHT DANA 257 



Dana has illustrated his conception of geographical evolution 

 by the concrete example of the development of the continent of 

 North America. 1 The continent is pictured at the beginning of 

 Paleozoic time, showing a V-shaped area of land composed of 

 Archaean rocks, with the apex of the V in the region of Lake 

 Superior, and its arms extending northeastward to Labrador and 

 northwestward to the Arctic Ocean, while other linear areas of 

 Archaean rock (protaxes) mark the positions of the Appalachian 

 chain on the east and the Cordillera on the west. Between the Ap- 

 palachian and the Cordilleran protaxis lies a vast Mediterranean 

 sea of shallow water (the Mississippian Sea, as it has been appro- 

 priately named), in which sedimentary deposits are gradually 

 accumulating, while its northern shore-line along the Archaean V 

 moves gradually southward, as the progressive oceanic subsidence 

 allows strip after strip of dry land to emerge and to be annexed 

 to the primitive nucleus of the continent. So the Paleozoic strata 

 crop out in parallel bands through New York and westward. 

 The tranquil progress is interrupted by the Taconic revolution 

 (post-Ordovician), uplifting a mountain range in western New 

 England and eastern New York, and probably other ranges now 

 in ruins farther south; and later by the Appalachian revolution 

 (post-Carboniferous), uplifting the Appalachian range from the 

 Catskills to the mountains of Alabama. The eastern half of the 

 continent becomes permanent dry land at the close of the Paleo- 

 zoic, while the evolution of the western half a newer territory 

 geologically as well as politically goes on through later time. 

 The Sierra revolution (post- Jurassic) and the Laramide revolu- 

 tion (post-Cretaceous), uplifting respectively the Sierra Nevada and 

 the main ranges of the Rocky Mountains, serve as time bound- 

 aries for the later ages of geological time, as the Taconic and the 

 Appalachian revolutions for earlier ages. 



The picture is a noble one, and in its main outlines true, though 

 the actual history was less simple than the student would naturally 



1 Notably in his presidential address before the American Association for 

 the Advancement of Science, in 1855; most fully, of course, in the Manual of 

 Geology. 



