126 Belemnitella Americana and Mucronata [324 



paratively shallow basin, very often not more than 25 fathoms 

 in depth. So with the great European chalk deposits. Their 

 purity is due rather to clear, shallow- water conditions, condi- 

 tions which a flat, peneplained adjoining land-mass would af- 

 ford, than to the clearness of abyssal oceanic waters. The 

 depth, or the distance from shore at which these conditions 

 would prevail, would be a function of the height of the land, 

 strength and direction of currents, mouths of rivers, etc. This 

 would suggest about the limit of the continental shelf, under 

 conditions as they exist today, but in the Cretaceous it is prob- 

 able that these existed at much shallower depths, for the 

 reasons given. 



In the case of the glauconitic facies of the American hori- 

 zons, considerably more difficulty is experienced in reaching 

 a definite idea as to the conditions of deposition, due to the 

 lack of knowledge on the formation of glauconite. There 

 has been no controversy comparable to that over the chalk. 

 The consensus of opinion seems to place the areas over which 

 deposition is going on today at about the edge of the con- 

 tinental shelf, or at slightly shallower depths. Goldman (4, 

 p. 176-82), has a discussion of the glauconite of the Monmouth 

 formation of Maryland, but does not state at what depth it 

 was formed, although he says it is found at 91 meters in the 

 North Atlantic today (4, p. 176). Sir John Murray says the 

 glauconite, which is usually found in shells of Foraminifera 

 when primary, -is always associated with terrigenous material. 

 The admirable work of L. Cayeux (14), discusses rather the 

 method of formation of the mineral glauconite than the physi- 

 cal conditions under which it was deposited. 



F. W. Clarke (18, p. 135), summarizes these as follows: 

 glauconite "is widely disseminated upon the sea-bottom, but 

 most abundantly in comparatively shallow waters, and near 

 the mud-line surrounding continental shores," that is, "just 

 beyond the limits of wave and current action, or in other 

 words where the fine muddy particles commence to make up 

 a considerable portion of the deposits." This is usually placed 

 at or about the edge of the continental shelf, or from 80 to 100 



