1895.] NATURAL, SCIENCES OF PHILADELPHIA. 153 



the greater portion of its water and at the same time damming back 

 the rivers allowed the fresh water to gradually spread over the whole 

 area and form a fresh water lagoon into which the rivers and streams 

 in time of floods carried great quantities of mud and fine sand. The 

 deposition of these in the still waters of the lagoons gradually formed 

 the sandy clays found sparingly distributed among the sanely de- 

 posits. But as the whole work of the sea is an incessant building up 

 and tearing down, a period of storms or extra high water would 

 eventually destroy the sandy barriers and again the whole flat would 

 be subject to marine influences and the deposition of marine deposits. 

 Probably the rivers themselves, by the washing out of the bars at 

 their mouths aided in the work. 



Although the land had gradually subsided, the subsidence scarcely 

 kept pace with the upbuilding of the sea and each succeeding inroad 

 would have less influence than the preceding, until a time would be 

 reached when the rivers and streams gained the ascendancy and then 

 would commence a time of heavier and more extensive clayey de- 

 posits. Most of "these clayey deposits are laminated and* from this 

 we may infer that they were never absolutely beyond marine influence, 

 however weak it may have been at times. 



At that time, we may reasonably suppose, the sea had formed a 

 broad bar of sufficient strength to prevent the ingress of salt water 

 except on very exceptional occasions or at low points in much the 

 same manner as along the present coast. No doubt there were some 

 differences but these we are unable to point out as all evidences have 

 long since been covered up by overlying deposits or erosion. 



It was to this time we may ascribe the beginning of the deposition 

 of the vast beds of lignite so prominent throughout the greater por- 

 tion of the area. Prior to this no lignite appears to have been 

 formed and none have been found in the lower sands. It must be 

 remembered that throughout the whole lignite field the lignites rest 

 upon clays and that these clays contain great quantities of carbo- 

 naceous matter in the forms of leaves and stems of delicate plants, 

 none of which could bear transportation to any great distance. 



Many conditions of origin have been ascribed to these lignitic de- 

 posits. Some writers assert " that these beds were formed as off-shore 

 deposits and the beds of lignite are accumulations of land vegeta- 

 tion carried out to sea and becoming water-logged, sunk in heaps in 

 much the same manner as beds are forming in the bed of the present 

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