POTOMAC FOKMATION IN VIlKilNIA AND MAKYLAND. 597 



Potomac time in Maryland, the District of Columbia, and \'ii-<i;inia c(jn- 

 sisted of several types, perhaps g;enerically distinct, l)ul still practically 

 sequoian. These forests, as the pi'cseiit work clc;iily shows, extended 

 entirely across the continent and piohahly covered the whole of North 

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

 the elements necessaiy to I'esist the chanfjes takinj; place in the 

 environment, especially the competition of the more modeiii conifeious 

 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 heen doniin.mt, and was finally stranded in two 

 narrow belts in California, along the ('oast Range and the Sierra Nevada, 

 respectively, where the last survivors of the genus Se([uoia still persist 

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

 redwood, and S. Washingtoniana (Winslow) Sudw'orth, the mammoth 

 tree. 



COLUMNAR SECTION OK THE I'OTOMAC K< »HM ATK )N . 



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 pii])lished 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 

 jis 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 iieds. I']xcept at Rosiers 



aFifteenth .\nii. Kept. U. S. Oeol. Surv., 1893-fM, p. 339. 



