THE CRETACEOUS PERIOD. 139 



marl, the last being rather characteristic of the system. The beds 

 of sand and clay are mainly unindurated, and do not differ notably 

 from other sedimentary beds of similar materials. Of limestone, 

 there is but little on the Atlantic coast, but more about the Gulf. 



A chief constituent of the greensand marl is glauconite, primarily 

 a hydrous silicate of potash and iron, 1 occurring in grains. Glauconite 

 is now making in some parts of the sea, and from the positions in which 

 it occurs, the following are inferred to be the conditions necessary for 

 its origin: 2 (1) water of moderate depth, 100 to 200 fathoms being 

 the most favorable; (2) a meagre supply of land-derived sediment, 

 and (3) the presence of foraminifera. The production of the glau- 

 conite seems to be effected by chemical changes induced in the sedi- 

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

 tained in the foraminiferal shells. The greensand marl of the Cre- 

 taceous system is somewhat widely distributed along the Atlantic 

 coast, showing that the conditions for its origin were widespread. 

 Since it is sometimes in distinct beds, separated from one another by 

 formations of other composition, there must have been a recurrence 

 of the conditions necessary for its origin; but even in those parts of 

 the system where clay and sand predominate, glauconite is not gen- 

 erally altogether absent. Similar marls are found in the Cretaceous 

 of Europe and in New Zealand, though in Europe they occur in the 

 Lower system as well as the Upper. The abundance of greensand 

 marl — not a common formation outside the Cretaceous — in the corre- 

 sponding systems of different continents, adds another to the many 

 striking inter-continental resemblances. 



The Cretaceous formations of the Atlantic coast have certain pecu- 

 liarities of structure, especially in that some of the beds when traced 

 along the strike, wedge out in one direction or the other. The suc- 

 cession of thin beds of unlike constitution shows that the conditions 

 of sedimentation were subject to numerous changes in the course of the 

 period. These changes may have been the result of changing depths 

 of water, changing heights of adjacent land, or of changing currents 



1 Glauconite is usually impure, and, as it occurs in nature, contains several other 

 ingredients. 



2 For brief summaries concerning the origin of greensand marl, see Clark, Jour, 

 of Geol., Vol. II, p. 161, and Reports of the State Geologist of New Jersey, 1892. For 

 fuller accounts, see Challenger Report on Deep Sea Deposits. 



