346 W. B. CLARK — UPPER CRETACEOUS FORMATIONS OF NEW JERSEY. 



surface persist. The records are not altogether satisfactory as regards the 

 subdivisions, however, but it seems highly probable that they change 

 materially and often entirely disappear along the line of dip. 



Variations of considerable magnitude in the deposits of the upper 

 Cretaceous of the northern Atlantic Coastal plain are found, as above de- 

 scribed, along both the lines of strike and dip, but the chief divisions 

 can everywhere be recognized throughout the area. As they are the only 

 divisions which can be thus employed throughout the region, they have 

 been given formational importance. 



PROBABLE MARINE CONDITIONS AS REVEALED BY THE DEPOSITS. 



The descriptions of the formations which have been given in the pre- 

 ceding pages show that the upper Cretaceous is chiefly composed of de- 

 posits in which glauconite is more or less commonly present. A knowl- 

 edge of the marine conditions can therefore be gained by instituting a 

 comparison between the deposits of the upper Cretaceous and those in 

 which glauconite is being formed at the present time. 



Great light has been thrown upon the origin of greensand deposits as 

 a result of the deep sea dredgings which have been made in recent years 

 by vessels sent out under national auspices. The most important of 

 these expeditions was that of the " Challenger," sent out by the British 

 government in the years 1872-76. In the report upon the deep sea 

 dredgings published as a result of that expedition Messrs Murray and 

 Renard, the authors, present the latest results upon the character and 

 distribution of greensand, and at the same time propose a theory to 

 account for the chemical changes which have taken place to produce the 

 mineral glauconite which characterizes all greensand deposits. 



A typical greensand, such as has been described in most of the Upper 

 Cretaceous formations, is composed of glauconite associated with greater 

 or less amounts of land-derived material, composed of the more common 

 rock-building minerals, together with fragments of the rocks themselves, 

 while to these is commonly added a variable amount of calcareous matter 

 derived from the shells of organisms. 



The greensand deposits of the present day are estimated. to cover a 

 million square miles of the sea-floor and are found limited to those areas 

 adjacent to the coast, and for the most part along the higher portions of 

 the continental slopes, where land-derived materials are deposited in 

 perceptible, yet small amounts. The " Challenger " dredgings show that 

 the production of glauconite seldom reaches to greater depths than 900 

 fathoms, and most commonly takes place between 100 and 200 fathoms, 

 although under favorable conditions it may be produced at shallower 

 depths. Its formation is interfered with by t\xe entrance of large rivers 



