744 GEOLOGY 



the conditions necessary for its development are the following: 1 

 (1) Water of moderate depth, 100 to 200 fathoms being the most 

 favorable; (2) a meager supply of land-derived sediment; and (:$) 

 the presence of foraminifera. The production of the glauconite 

 seems to be effected by chemical changes induced in sediments 

 perhaps as the result of decomposition of the organic matter con- 

 tained in the foraminiferal shells. The abundance of greensand 

 marl, which is not a common formation outside the Cretaceous, 

 in corresponding systems of different continents, is one of the 

 many striking inter-continental resemblances. 



The aggregate thickness of the Cretaceous beds along the Atlan- 

 tic coast nowhere exceeds a few hundred feet. 



The subdivisions now generally recognized are the following, 

 commencing with the lowest: 1. Matawan formation, 2. Mon- 

 mouth formation; 3. Rancocas formation; 4. Manasquan formation. 



These formations are not severally continuous throughout the 

 coastal region. Thus the Matawan formation does not appear at the 

 surface 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 of the outcrops 

 show that they also vary seaward from their landward margins. 



Though the beds have been but little changed since their depo- 

 sition, certain alterations are worthy of note. The porous beds of 

 greensand marl have been changed, locally, to brown, by the decom- 

 position of the silicate and the formation of ferric oxide. Cemen- 

 tation, chiefly by ferric oxide, has indurated certain beds at some 

 localities, and many of the conspicuous hills within the area of 

 Cretaceous outcrops are due to a capping of this ironstone. The 

 cemented layers are most likely to occur at the junction of forma- 

 tions of different texture, a generalization which holds in otl im- 

 partially indurated series. 



The Eastern Gulf Border 



The Cretaceous formations of the Eastern Gulf states ap] 

 at the surface some distance from the coast (Fig. 505), ;ml 



1 For brief summary concerning the origin of gnvnsaiul m.-irl. see Cl 

 Jour. Geol., Vol. II, p. 161. For a fuller account, see Challenger Report 

 Deep Sea Deposits. 



