GEULOGY OF Till-: POTOMAC BEDS. 47 



directly on the Potomac, it is not always easy to distinguish the Potomac 

 gravels from tlie later ones. In tliis material the Potsdam cobbles are the 

 most signilicant, and they are usually ea.^ily identified. The Potsdam 

 gravel is not seen north of Washington, and south of that city it is not 

 traced farther than Pohick Church, near White IIou.se Point. This 

 distribution indicates that these stones were Ijrought down the Potomac 

 River, which crosses for the last time outcrops of the Potsdam a little we.-t 

 of Harper's Ferry. The Potsdam cobbles may be seen throughout the 

 Petersburg area near the outcrops of the Potomac, and they are found, 

 as near Washington, over a belt of country extending considerably to 

 the west of it and witliin the Azoic- terrane. Thev may be followed also 

 all along to the west of the Hanover area, and five or si.x miles to the 

 north of its termination. Much rounded quartz material is found with 

 these stones. In the Azoic terrane they are often found inclosed in Appo- 

 mattox cla3's and sands. 



The distribution of this coarse, loose material indicates that the 

 Potomac once extended west of its present exposures over a belt varying 

 from ten to twenty miles in width. 



THE GEOLOGY OK THE POTOMAC liED.S. 



So uuich has been said in the preceding pages concerning the geology 

 of the formation, that the remarks under this head ma}- Ije much abbre- 

 viated. It is very difhcult in any Ijrief general account to give a clear 

 idea of the character of this formation. The want of regular structure 

 and the continual, often abrupt, changes in mineral character and pln'sical 

 features make it necessary to give a separate description of each locality. 

 Sections at the same locality, taken at intervals of a few )-ards or even feet, 

 vary generally in most of their features. It is impossible to take the dip 

 and strike, and no single bed can be traced over anv large area. This 

 arises from the fact that the characteristic mode of arrangement of tlie 

 materials is in irregular packets, or more commonly in interlocking 

 lenticular layers. The layers may each continue throughout, containing 

 the same kind of material, dovetailing between la3'ers of a very dillerent 

 kind. A layer, on being traced horizontally or vertically, may pass 



