68 TEANSACTIONS OF THE 



[1889 



It is interesting to notice how different has been the action of 

 atmospheric agencies in modifying the condition of the fossils on 

 the opposite sides of the Western Shore. In the region lying 

 between the Potomac and Patuxent rivers the great proportion 

 of these fossil shells has been scarcely affected by the iron solu- 

 tions which stain the soils ; while in the country extending 

 from South river to the Magothy, only a very small number of 

 the same species can be found otherwise than deeply penetrated 

 by the iron brown solution, and reduced to a much more inco- 

 herent condition by its solvent tendency. While, however, the 

 iron eats the shell completely away from the casts of fossils in 

 many localities, in others it has the good quality of cementing 

 the matrix of greensand marl which contains the shells, and by 

 fastening all together in large masses, produces a rock-like 

 body possessing great resistance to atmospheric and corrasive 



agencies. 



Another condition of the fossils is that in which they appear 

 only as slightly coherent casts, covered and held in form by a 

 coating of iron film. These soft bodies occur distributed sparsely 

 throughout the black sandy marls of the cliffs and gullies on 

 Port Tobacco Creek, while another variety rests in the clayey 

 greensand marls which overlie the shell rock in the vicinity of 

 Upper Marlboro, and in various parts of Anne Arundel 

 county. These fossils have been reduced to their present con- 

 dition by caustic and acid agents soaked through the soil in 

 aqueous solutions. Accordingly no part of the shell is left 

 remaining, and the fossil relics are obtainable only in the most 

 tender condition. 



A similar solidification also takes place in the remains of 

 fossils which occur in drifts or layers in the whitish sandy loam 

 overlying the black marl on Port Tobacco Creek. These, then, 

 are the principal modifications observed in the shells which be- 

 long to the Eocene of the State of Maryland. 



