140 GEOLOGY. 



in the water. The frequent variations in the character of a stratum when 

 traced laterally show that different conditions prevailed at different 

 points along the coast at the same time. 



Thickness. — The aggregate thickness of the Upper Cretaceous beds 

 nowhere exceeds a few hundred feet. In New Jersey it is about 500 

 feet, and in Maryland 200. 



Classification. — Various classifications have been in use for the Cre- 

 taceous formations of the Atlantic coast, that now generally adopted 

 being as follows : 1 



4. Manasquan formation. 

 3. Rancocas formation. 

 2. Monmouth formation. 

 1. Matawan formation. 



These formations are not severally continuous throughout the Coastal 

 region. Thus the Matawan formation does not appear at the sur- 

 face south of Maryland, being overlapped in that direction by later 

 beds. All the formations show notable variations when traced along 

 their strikes, and borings to the east show that they also vary when 

 traced to seaward from their landward margins. 



Changes in the beds since deposition. — Though the beds have 

 been but little changed since their deposition, the slight alterations 

 are worthy of note. Locally, the porous beds of marl have been changed 

 from green to brown by the decomposition of the silicate and the forma- 

 tion of ferric oxide. Cementation, chiefly by ferric oxide, has indurated 

 certain beds at some localities, and many of the conspicuous hills 

 within the area of Cretaceous outcrops owe their existence to a capping 

 of this ironstone. The cemented layers are most likely to occur at the 

 junction of formations of different texture, a generalization which holds 

 in other unindurated, or but partially indurated systems. The lime- 

 stone of the formation is often thoroughly indurated. 



The Gulf border region east of the Mississippi. — Along the Gulf coast, 

 as along the Atlantic, the Cretaceous beds appear at the surface some 

 distance from the coast, and dip seaward at a low angle. The belt 

 of their exposure extends from Georgia on the east, through Alabama 



1 Clark, Bull. Geol. Soc. of Am., Vol. VIII, p. 326. See Repts. of the State Geol- 

 ogist of N. J. for older classification. The subdivision of this system, as proposd b}' 

 Clark, has been somewhat modified by Knapp and Weller, Jour, of Geol., Vol. XIII, 

 pp. 71-84. 



