320 TRANSACTIONS OF THE 



At Harper's Ferry, and extending for 10 miles east- 

 ward, occurs a series of modified rocks, whose characters 

 in many respects assimilate to those which we had tra- 

 versed near Paris and the Blue Ridge, and which have 

 been described in the preceding article. 



Passing these we intersect the calcareous breccia 

 known by the name of Potomac marble, and a belt of 

 blue transition limestone. The remainder of the section 

 consists of a variety of the inferior stratified rocks. 



I'OSSIL PLANTS IN THE SECONDARY HORIZONTAL 

 STRATA OF FREDERICKSBURG. 



The conglomerates, grits, and gritty argillaceous beds 

 constituting the groupe which we have described under 

 this appellation, have been held by one writer to be 

 identical with the testaceous beds at Annapolis ; but as 

 those are calcareous and probably tertiary, and as their 

 connection has not been traced, the supposition appears to 

 be unauthorized. Opposite Fredericksburg this groupe 

 rises to the height of 80 feet, and near Falmouth 100 

 feet above the river, while near its western termination 

 it attains an elevation of probably near 150 feet above 

 tide level. This rock is a true depository of lignites, 

 silici&ed masses of wood, the fragments of large trees, 



Black Limestone. Grey variety of Limestone. 



Alumina, 



15.2 



Alumina, 



32.6 



Magnesia, 



08.2 



Magnesia, 



02.6 



Oxide of iron, 



01.4 



Oxide of iron. 



02.0 



Carbonic acid, water, bitumen. 



39.2 



Carbonic acid and water. 



29.8 



Lime, 



36.0 



Lime, 



33.0 





100.0 





100.0 



Blue Limestone.. 



Alumina, .... 04.8 



Lime, .... 51.2 



Carbonic acid and water, . 41.4 



Other substances, oxide of iron, (fee. 02.6 



100.0 



