POTOMAC FORMATION IN VIRGINIA AND MARYLAND. 597 



Potomac time in Maryland, the District of Columbia, and Virginia con- 

 sisted of several types, perhaps generically distinct, but still practically 

 sequoian. These forests, as the present work clearly shows, extended 

 entirely across the continent and probably covered the whole of North 

 America. But for some reason the sequoian type of structure lacked 

 the elements necessary to resist the changes taking place in the 

 environment, especially the competition of the more modern coniferous 

 vegetation that came on in later Cretaceous and Tertiary time, and it 

 was gradually crowded out of existence over most of this great area 

 where it had so long been dominant, and was finally stranded in two 

 narrow belts in California, along the Coast Range and the Sierra Nevada, 

 respectively, where the last survivors of the genus Sequoia still persist 

 in the only two living species, <S. sempervirens (Lambert) Endlicher, the 

 redwood, and S. Washingtoniana (Winslow) Sudworth, the mammoth 

 tree. 



COLUMNAR SECTION OF THE POTOMAC FORMATION. 



Taking into consideration all the facts presented in Professor Fon- 

 taine's report as condensed in the table, together with all that was known 

 of the Potomac formation down to the present time, it is possible to 

 recast the section of the entire formation. This, then, will assume some- 

 thing like the following form : 



In the geological column published in my paper on the Potomac 

 formation" I gave the entire formation a thickness of 1,175 feet. If 

 we now give it a thickness of 1,200 feet, which it probably has, and make 

 the Raritan, as was done then, 500 feet, we have for the Older Potomac 

 a total thickness in Maryland of 750 feet, of which the upper 225 feet 

 are not represented in Virginia. This is the portion to which I then 

 assigned the iron ore, under the prevailing impression that all the Mary- 

 land beds were higher than any of the Older Potomac in Virginia. We 

 now know that practically all the iron ore occurs on the same horizon 

 as the Rappahannock of Virginia, viz, in the Arundel of Clark and Bibbins. 

 These beds in Maryland overlying the iron-ore clays and assigned to the 

 Patapsco consist of alternating clays and sands and form a more or less 

 gradual transition into the overlying Raritan beds. Except at Rosiers 



^Fifteenth Ann. Kept. U. S. Geol. Surv., 1893-94, p. .3.39. 



