206 CLARK AND BIBBINS — GEOLOGY OP THE POTOMAC GROUP 



trunks are at times found erect with their roots intact, find their most 

 satisfactory explanation on this basis. It was in these ancient marshes 

 that the iron, derived to a considerable extent from the adjacent area of 

 basic eruptives, was deposited, first, no doubt, as bog ore, which by con- 

 tact with lignite was later altered to the carbonate and redeposited in its 

 present nodular form. It was in these marshes that the remains of 

 dinosauria became entombed, which, with the evidence of dense vegeta- 

 tion, suggests subtropical climate. 



On this hypothesis the lenses of Arundel clays, particularly to the 

 landward, represent crudely the ancient drainage lines of the eroded sur- 

 face of the Patuxent terrane. The widening of the areas seaward may 

 possibly be interrupted on the basis of lagoon deposits into which the 

 Arundel estuaries merged. That the waters in which the Arundel de- 

 posits were laid down were not entirely cut off from the sea is evidenced 

 by the occasional occurrence of Teredo-bored conifers, while the absence 

 of strictly marine fossils suggests that the Arundel waters had but imper- 

 fect marine communication. % 



It has been suggested by the students of the Maryland Pleistocene 

 that the " buried-forest " deposits of the Chesapeake shores may furnish 

 some clue to the origin of the Arundel iron ore clays. These deposits 

 appear to have originated by the impounding of the estuaries by sand- 

 spits — a process which may be observed at many points within the 

 Chesapeake and elsewhere at the present day. The closed estuary then 

 speedily silted up and was converted into a peaty cypress swamp, in 

 which bog iron ore was deposited. Meanwhile the bay shore adjoining 

 the mouth of the swampy estuary was gradually receding by virtue of 

 wave action until the swamp materials themselves were invaded and 

 more or less cut away. This process was followed, or perchance at- 

 tended, by gradual subsidence, which resulted in the deposition on the 

 newly wave-cut surface of a new and later member. Emergence fol- 

 lowed, and the waves are now actively cutting away both the more re- 

 cently deposited terrane and the basal remnant of the older one, with its 

 beheaded cypress trunks and their knees, imbedded in peat. In the 

 basal clays of the swamp deposit, penetrated by the roots of the trees, 

 one finds an occasional, imperfectly formed nodule of iron carbonate. 

 When exposed to the air it rapidly changes to a bright vermilion ocher. 



There is little question that some such process as this has figured to a 

 considerable extent in the genesis of certain of the lesser lenses of drab, 

 lignitic, iron-bearing clay occurring at various horizons throughout the 

 Potomac group ; but the grand scale — both vertical and horizontal — on 

 which the Arundel formation, or " iron-ore clays " proper, were laid down 

 cannot well be explained entirely by this simple theory. The idea of 



