4 STRATIGR.\PHICAL GEOLOGY. 



rocks, and the consequent method in which the continent has been grad- 

 ually built up, I have prepared a small geological map, which will exhibit 

 these mutual relations far better than words. The projection is the ordi- 

 nary one, exhibiting the surface as nearly as possible like nature, and not 

 distorting the northern regions so fearfully as Tvlercator's. The forma- 

 tions for the United States are copied in outline from the geological map 

 by W. P. Blake and myself, published in the supplementar}' atlas pertain- 

 ing to the ninth census, Gen. F. A. Walker, superintendent. The delin- 

 eation of the northern portion is taken mainly from a map by Robert 

 Bell, in Walling's topographical atlas of the dominion of Canada, and 

 from notes respecting Alaska, kindly furnished me some years since for 

 another purpose by W. H. Dall. As the scale is so small, the absence 

 of many considerable outcrops, especially in the Rocky Mountain region, 

 is a matter of necessity. 



An inspection of our map will show a natural division into the follow- 

 ing grand districts, the most important being stated first, without regard 

 to age: i. The immense Rocky Tvlountain crystalline area, extending 

 from Alaska into Mexico. This is the foundation for innumerable minor 

 basins, which cannot be represented. 2. An equally extensive crystalline 

 area, embracing the north-eastern section of the continent, chiefly in the 

 territories of Canada, Labrador, and Greenland. This portion is rather 

 quadrangular in shape, with a central depression for the Hudson's Bay 

 Paleozoic area. 3. The interior Paleozoic basin, nearly divided along the 

 middle so as to separate between the arctic and eastern United States 

 regions. 4. A somewhat similar Mesozoic region lying to the west of 

 the preceding, and broadest southerly. 5. The elevated fresh-water Ter- 

 tiaries of the upper Missouri, and the marginal Cenozoic groups on the 

 Atlantic, Gulf of Mexico, and the northern waters off Alaska. 6. The 

 Atlantic belt of Eozoic formations. 7. Various local basins, as the Pale- 

 ozoic of Acadia and of the Colorado river ; the Mesozoic of Vancouver's 

 island, California coast range, of Utah and New Mexico, and of Alabama 

 and Mississippi; Cenozoic of Rocky Mountains, Greenland, etc. It is 

 the sixth of these districts that we shall call particular attention to 

 very soon. 



I cannot resist calling attention to a very few points suggested by 

 this map : First. At the close of the Eozoic period the grand features 



