MARINE CONDITIONS AS REVEALED BY DEPOSITS. 347 



bearing sediment into the sea and by the prevalence of strong oceanic 

 currents. 



It is a remarkable fact that although greensand is not formed except 

 in the presence of land-derived materials its production is accomplished 

 through the intervention of foraminifera, and brought about by chemical 

 changes which take place in the finely comminuted sediment as the re- 

 sult of the decomposition of the organic matter inclosed in the shells and 

 disseminated in the surrounding mud. Glauconitic casts of foraminifera 

 are of common occurrence in such deposits. 



It will be observed, then, that two conditions are requisite for the pro- 

 duction, of glauconite. First, the deposition of mineral particles of land- 

 derived origin ; and, second, the presence of foraminifera. In the absence 

 of either, the production of greensand evidently does not take place, 

 while its formation is retarded and finally ceases altogether as the amount 

 of deposited materials increases adjacent to the coast. 



The conditions for the formation of greensand being then as above 

 described, it is probable that the succession of events during the upper 

 Cretaceous along the northern Atlantic coast was somewhat as follows : 

 With the opening of the Matawan epoch moderately quiet, deep seas pre- 

 vailed over most of the area, resulting in a slow accumulation of muddy 

 sediments, in which locally and for brief periods the conditions were 

 favorable for the formation of glauconite. Later in the Matawan epoch 

 the conditions of sedimentation changed in the north, but remained 

 much the same in the south. Thick bedded sands were laid down over 

 the northern portion of the area, although a return to the muddy sedi- 

 mentation occurred prior to the close of the epoch throughout a por- 

 tion of the district, bringing with it locally conditions again favorable to 

 the production of glauconite. The epoch closed in the north with a re- 

 newed deposition of sand, at this time, however, highly micaceous, the 

 micaceous materials also characterizing the finer deposits of the south, as 

 they had done to some extent the sediments of that area throughout the 

 epoch. 



With the advent of the Monmouth epoch land-derived materials were 

 largely increased in volume in the southern portion of the district, but 

 were only deposited for a short time in the north, where they were shortly 

 succeeded by conditions highly favorable for the production of glauconite, 

 with every indication of quiet and deep seas. These conditions, how- 

 ever, were less pronounced in proceeding southward, and over the area 

 of southern Maryland sedimentation similar to that which had char- 

 acterized the earlier portion of the epoch was continued. The epoch 

 closed with the deposit of a great volume of sands and clays, sparsely 

 glauconitic, throughout northern New Jersey as well as in Delaware and 



UI— Bull. Geol. Soc. Am., Vol. 8, 1896 



