CEETACEOUS PERIOD. 409 



many of the Basin ranges were formed at this time (King), although 

 their present forms were given much later (p. 265), It was essentially 

 a period of mountain-making in the western part of the American 

 Continent. 



Section 4. — Cretaceous Period. 



The most general characteristic of this period is its transitional 

 character. In it Mesozoic types are passing out, and Cenozoic or 

 modern types are coming in, and the two types therefore coexist side 

 by side. Nearly everywhere in America, as far as known, the Creta- 

 ceous lie unconformably on the Jurassic or still lower rocks. • 



Rock-System — Area in America. — 1. On the Atlantic border going 

 southward, we find no Cretaceous rocks until we reach New Jersey. 

 Here we find a small patch peeping out from under the edge of the 

 overlying Tertiary, and marked on the map (p. 291) by oblique inter- 

 rupted lines. This patch passes through New Jersey, Delaware, Mary- 

 land, to the borders of Virginia. Passing south, we find no continuous 

 area until we reach Georgia ; yet it underlies the Tertiary in all this 

 region, as is shown by the fact that the rivers in North and South 

 Carolina cut through the Tertiary and expose the Cretaceous in many 

 places. ~ 2. The Gulf-border Cretaceous commences in Western Middle 

 Georgia, covers all the prairie region of Middle Alabama, the northeast- 

 ern or prairie region of Mississippi, then runs northward as a narrow 

 strip through Tennessee nearly to the mouth of the Ohio. It then dis- 

 appears beneath the Tertiary, to reappear as an area bordering the Gulf 

 Tertiary on the west side. 3. On the interior plains, the Cretaceous 

 connecting with the Gulf -border area stretches northwestward to arctic 

 regions, occupying nearly the whole of the great, grassy, level Western 

 Plains called Prairies — though much of it is overlaid by the subsequent 

 Tertiary. 4. In the Rocky Mountain region Cretaceous strata occupy 

 all the Plateau region — i. e., the region between the Eastern range and 

 the Wahsatch range, except where overlaid by Tertiary or removed by 

 erosion. Recent investigations in Mexico * render it probable that this 

 area stretches also westward through Northern Mexico to the Pacific. 

 5. On the Pacific border, Cretaceous strata form a large part of the Coast 

 Ranges, and also in places the lowest western foot-hills of the Sierra 

 Raiige. Whitney has estimated the thickness of the Cretaceous rocks 

 in portions of the Coast Range as 20,000 feet, and Dillon, 30,000 feet. 



Physical Geography in America.— It is not difficult from the Creta- 

 ceous area just given to reconstruct approximately the physical geog- 

 raphy. At that time the Atla?itic shore-line in all the northern portion 

 of the continent was farther out or east than now, for the Cretaceous 



* American Journal of Science, vol. x, p. 386, 1S75. 



