348 HISTORICAL GEOLOGY. 



was a downward movement of the crust over the whole 

 Plains and Plateau region hy which isolated inland seas 

 were changed into the great interior Cretaceous sea. 

 The Sierra Nevada Range is the most conspicuous monu- 

 ment of this period of change, and therefore it may be 

 called the Sierra revolution. 



SECTION IV. CRETACEOUS ROCKS AND PERIOD. 



General Characteristics. The Cretaceous is in some 

 respects a transition to, and a preparation for, the next 

 era. Mesozoic types, such as the great reptiles, the am- 

 monites, etc., continue, but Cenozoic types, like dicoty- 

 ledonous trees and teleost fishes, are introduced, and the 

 two kinds of types coexisted side by side. 



Rock System ; Areas. 1. In the Atlantic border 

 region, going southward, we find no cretaceans until we 

 reach Long Island. Going south from this, we find a strip 

 running through New Jersey, Delaware, and Maryland, 

 lying directly against the Archaean ; then small, isolated 

 patches exposed by erosion in North Carolina, South 

 Carolina, and Georgia. It doubtless extends all along 

 the Southern coast, but is mostly covered with later Ter- 

 tiary deposits. 2. In the Gulf border region it forms a 

 broad, crescentic band, commencing in western middle 

 Georgia, passing through middle Alabama, turning north- 

 ward through Mississippi and Tennessee, to near the 

 mouth of the Ohio. It underdips the Tertiary of the 

 Mississippi River region, and reappears on its west side 

 (see map, page 272). 3. It thence passes northward, 

 covering nearly the whole Plains and Plateau region, 

 though largely concealed by the Tertiary. 4. On the 

 Pacific border it is found on the lower foot-hills of the 

 Sierra Nevada in Northern California, and, together with 

 the Tertiary, forming the whole of the Coast Range. 



Physical Geography. From this distribution we can 



