52 TRANSACTIONS OF THE [1889 



the bliiif about one mile above Fort Washington, on the Piscat- 

 away. Tlie general uniformity of the materials constituting a 

 stratum in each part of the territory shows how the same forces 

 were at work in restricted localities. The finely reduced char- 

 acter of the particles of clay, sand, and mica in the plastic black 

 marl strongly contrasts with the coarse sandy composition of 

 the beds in the other localities, such as we see on Round Bay, 

 and near Waterbury, in Anne Arundel county. 



It seems likely that the upper member of these greensand 

 marls has been to a large extent worked over by triturating 

 waters so as to reduce it at length to extreme fineness of par- 

 ticles. If original sedimentation alone had been the factor in 

 composing these beds, it is difficult to see how such uniformity 

 should prevail throughout so much of their area. Laid down 

 as these beds are, not simply in holes at the bottom of a deep 

 ocean, but spread over the slope of a coast whose border was 

 partly indented by estuaries, we should expect to find a sorting 

 of the materials from the coarsest to the finest progressively in 

 the direction of the open ocean. Such, however, does not prove 

 to be the case, and we are accordingly driven to the conclusion 

 that an almost landlocked oceanic tract existed, partly over and 

 partly outside of the Albirupean coast, into which the wash of 

 the adjacent land and precipitation of the dead bodies of minute 

 animals was continuously going on through a long period of 

 time. The lower member of this series must have derived most 

 of its material from the Albirupean forests, meadows, and sand- 

 hills ; while the upper strata might have been taken from the 

 clays at a higher level, and therefore farther inland, and ground 

 up with the sands from the adjoining hills, while the bodies and 

 shells of the animalculae were being steadily incorporated with 

 the accumulating mud. The top of the lower, or black, member 

 was denuded and eroded before the superincumbent strata were 

 laid down, and it now seems quite probable that the middle 

 and upper members of the series were not always deposited, 

 since they are absent from many parts of the territory, and 

 because there is no evidence of closely following denudation 

 having taken place in these localities. 



The great Clay Marl member, which forms such an extensive 



