200 TEANSACTIONS OP THE [1892 



summit. Lenticular beds, either short or long, composed 

 of the clay and drifted material of the adjoining region, 

 have canght and held the sands, clays, lignite, stems, 

 leaves and seeds of numerous varieties. Here are to be 

 found also lumps of clay and the fossils washed out of 

 older deposits belonging to the preceding formation. 



To the Albirupean I would refer the entire series of 

 clays, sands, sandy clays, white and pale sandstone and the 

 cobble-stone deposits and lenses of sedimentary materials 

 resting between the top surface of the " Variegated Clay " 

 and the base of the Green sand Cretaceous or Severn for- 

 mation. 



Alternate Clay Sands. — This group of sediments is re- 

 markable for the thick series of thin layers of clay and fine 

 white sand, including sometimes thicker beds of both the 

 clay and sand, and for a thick stratum, or line of beds, of 

 black argillaceous marl, placed at or near the top of the 

 group. 



This group is a unit of structure, resting upon the irregu- 

 lar and eroded surface of the pale or white clay, or sand, of 

 the preceding group, or first recorded series of the Albiru- 

 pean. All of its clays are more or less fossiliferous, and 

 the great black bed is in some places crowded with the re- 

 mains of trees and plants. It is not now possible to give 

 an extended list of the fossils, but from all the expo- 

 sures of these beds hitherto examined, either in Mary- 

 land or New Jersey, the same genera and species of fossil 

 plants have been obtained. 



The time-break between the Alternate Clay Sands and 

 the underlying group of the Albirupean is far less exten- 

 sive and profound than that at the close of the Potomac 

 formation, but it is distinct and will later reveal its more 

 complete significance. 



After the close of the Alternate Clay Sands Series, active 

 erosion, accompanied by dispersive and distributive forces, 



