814 HISTORICAL GEOLOGY. 



ceous period had made some progress, the waters of the Mexican Gulf 

 began to spread northward over the subsiding Continental Interior ; and 

 before its earlier half had passed, the submergence had reached its maximum. 

 A vast Mediterranean Sea extended from the inner portion of the Mexican 

 Gulf, probably to the Arctic Ocean. The Atlantic border, south of New York, 

 as the map shows, was also submerged, and a wider portion of the Gulf 

 border ; and along the valley of the Mississippi the waters stretched north- 

 ward beyond the present mouth of the Ohio, making a great Mississippi Bay, 

 which was 100 miles wide in the latitude of Memphis. But in Mexico, at this 

 time, the large Lower Cretaceous area over which the Atlantic and Pacific 

 had been exchanging waters, was, to a great extent, emerged. Over the 

 Pacific coast region there was a narrow strip of water — narrower than in 

 Jurassic time because of the making, at its close, of the Sierra Nevada and 

 other mountain ranges to the north. 



At the time of maximum submergence during the Upper Cretaceous, the 

 American Mediterranean Sea of the period had a length, if extending to the 

 Arctic Ocean, of about 3000 miles. The decline in depth and size began 

 perhaps by the middle of this later half of the Cretaceous period. As 

 explained in detail beyond, there were successively : a shallowing of the 

 sea and an emergence of dry land far north in British America, cutting off 

 connection with the Arctic Ocean, and thus converting the waters on the 

 south into a Mexican Gulf, 2000 miles or more in length ; a contraction of 

 the great gulf commencing along the eastern border ; the conversion of the 

 gulf, while still 1500 miles in length, into a region of alternating brackish 

 waters and fresh waters and low marsh-covered lands, situated along and just 

 east of what is now the Eocky Mountain section of the Western Continental 

 Interior ; the disappearance of the salt waters, leaving only fresh waters ; 

 • and, finally, the disappearance of these waters over the mountain region, 

 and the end of Cretaceous deposition, with a change in the events to moun- 

 tain-making. 



The preceding map, illustrating North America in the Cretaceous period, was pre- 

 pared for this work chiefly by J. S. Diller and T. W. Stanton, of the U. S. Geological Survey, 

 from the map by C. A. White in his paper On the Correlation of the Cretaceous ( JJ. S. 

 G. 8. Bulletin, No. 82, 1891), and from those for British America by G. M. Dawson in the 

 Transactions of the Boyal Society of Canada for 1890, and largely from later papers since 

 published, and more recent results in possession of the U. S. Geological Survey. White's 

 Bulletin, which contains a review of the literature, facts, and theories pertaining to the 

 North American Cretaceous formation, has been of much service in the preparation of the 

 following pages. 



Subdivisions. 



No general subdivisions of either the Lower or Upper Cretaceous for all 

 the regions in North America have been adopted, on account of the wide 

 diversity of the regions as to conditions. Part of the deposits being fresh 

 water, and the marine fossils of the Atlantic and Pacific borders and of the 



