Maryland Geological Suhvet 143 



very fine, and a more sudden drop to the fine, features which from the 

 diagrams on p. ]?0 are seen to differentiate open-water sediments from 

 those of deltas or streams (compare figs. A, C, E, with D, I, J, p. 170) . 



Furthermore, while at first sight the proportion of heavy minerals 

 shows no consistent difference in the two groups, it is found when glau- 

 eonite is deducted that the percentage of heavy minerals in the glauconitic 

 type is only 2%-3%, while in the " delta " type it is about 8%. Besides, 

 the deduction of glauconite is much more significant in the glauconitic 

 type, since here it is not an imported mineral. But while in its fresh con- 

 dition glauconite has generally a specific gravity considerably less than 

 2.7, it is questionable whether the material in bed 1 had not already 

 become partly decomposed, and thus actually a heavy mineral, before it 

 was transported into bed 1. In sample 3, which is the portion of bed 1 in 

 which glauconite is particularly amundant, both the glauconite itself and 

 the ocherous staining of other minerals support this belief, as I have indi- 

 cated in the discussion of that sample. 



As to the history of the succession that can now be worked out for this 

 section I should first question the field determination of a sharp contact 

 between beds 1 and 2. On the contrary, since bed 1 is very argillaceous, 

 and bed 2, while it grows more sandy towards the top, is also argillaceous 

 at its base, it seems more probable that there was here a transition, though 

 it may have been quite sudden. 



Then we have in bed 1 the evidence for the exposure of an older glau- 

 conitic bed to the atmosphere with partial decomposition of the glauconite 

 and ocherous staining of the other grains. This bed was attacked by the 

 stream which deposited in its delta the material of bed 1, while through 

 deepening of the water or reduction in grade of the supplying stream the 

 material gradually grew finer. Ultimately by a continuation of this evo- 

 lution the waters became quiet and clear, and favorable to the formation 

 of glauconite. Under these circumstances beds 2 and 3 (samples 5 and 6) 

 were formed, but the conditions controlling were, at noted above, not in 

 all respects similar for the two beds. A glance at E and F, p. 169, and com- 

 parison with the figures on p. 170 show at once the essential grouping of 



