1892] MARYLAND ACADEMY OF SCIENCES. 201 



cutaway the inner border of the Albirupean Black Marl and 

 made an inland troni^li, now partly occupied by Ilaritan 

 bay, the Delaware river, Chesapeake bay and the Severn 

 river, etc., while the adjacent country is now extensively 

 occupied by several of tlie beds belonging to the Greensand 

 Cretaceous formation. 



The bottom of the basal member of the Greensand for- 

 mation, in New Jersey, where present, as near Cheese- 

 quakes creek, is signalized by a thick, stony, ferruginous 

 crust, charged with the remains of fossil shells. The shells 

 are generally eaten away, so as to leave a hollow mould 

 where they formerly existed. But occasionally enough 

 of the shell remains to serve for identification. It has 

 accordingly been possible to recognize Ideonarca vulgaris 

 Morton, Ostrea larva Lamark, and an Axinea allied to H. 

 alta Whitf. 



From the foregoing observations it will be seen that a 

 great, and long extended, and wonderfully varied complex 

 of sedimentary formations, older than the Greensand Cre- 

 taceous and younger than the Triassic, composed a period 

 in the history of the seaward border of the Atlantic region 

 in late Jurassic, or early Cretaceous time. 



Su])2>lementary Ohservations. — Considering the several 

 members of the "Period" to which the Potomac and Albi- 

 rupean formations belong, the fact should be recognized 

 that, tlie group of deposits in Virginia, extending from 

 near Mount Yernon and back into the country to below 

 Fredericksburg, is a composite one, embracing numerous 

 kinds of materials derived from older beds, and put to- 

 getlier after a peculiar pattern of construction. Nothing 

 exactly comparable with this complex nnit of structure 

 exists in the other States of the Atlantic border, excepting 

 the small area which lies detached from the Potomac for- 

 mation and forms the hill near Indian Head, Maryland. 



