Oi THE EOCEXE DEPOSITS OF 3IAEYLAXD 



There are some exceptions to this general average, however, and a local 

 dip of 22 feet per mile was observed above Popes Creek. 



THICKXESS. 



It is evident, therefore, that on this basis of calculation the thickness 

 of the Eocene strata in this area must exceed 200 feet, although it will 

 probably fall short of 300 feet, the amount claimed by Darton. 



The results obtained from a study of the various section-lines and 

 well-borings show that the average thickness of the deposits is some- 

 Avhat more than 200 feet, although estimates based on the Potomac 

 river section, as well as on well-borings in the area to the east of Fred- 

 ericksburg, show that it somewhat exceeds that amount in this portion 

 of the Middle Atlantic Slope. 



The extensive covering of post-Eocene deposits in many portions of 

 the region renders it impossible to obtain data upon which an estimate 

 can be based, and the results of further well-boring will be awaited with 

 interest. From the facts already obtained it seems probable that there 

 is a slight thickening of the beds to the eastward along the line of dip. 



Oeigix' of the Mateeials. 



As regards their origin, the chief constituents of the Eocene deposits 

 of the Middle Atlantic Slope ma^' be grouped under three heads, viz.: 

 First, the arenaceous and argillaceous elements, which are land-derived: 

 second, the calcareous elements, which are of organic origin; and third, 

 the glauconitic elements, which are of secondary formation. 



The arenaceous and argillaceous materials were undoubtedly orig- 

 inally derived from the crystalline rocks of the Piedmont Plateau, with 

 an indefinite admixture from the Paleozoic formations of the Appala- 

 chian region. 



The organic remains, which consist very largely of the shells of mol- 

 lusks, are generally so slightly worn or broken as to Justify the belief 

 that they were little disturbed prior to their burial by the sediments in 

 which they are now found entombed. They have, however, lost con- 

 siderably by solution since they were deposited, the calcareous matter 

 removed serving as a cement to produce the limestone layers found at 

 several horizons in the southern Maryland and northern Virginia area. 



