342 MKSO/OIC FLORAS OF INrrFD STATES. 



Xeithoi' does he s(mmii to liav(> found any additional material of a vegetable 

 nature, and conte4its himself with enumerating the species described by 

 Professor Fontaine (pp. 165-166) and reproducing his figures of the most 

 striking of these (pi. xxvi). It therefore only remains to embody this work 

 in the literature of our suliject, and to draw attention to it as by far the 

 most complete account thus far given of the geolog>' of Texas. 



FT.OUA OF rilK OLDKU TOTOMAC FOITMATIOX. 



I shall use the term Older Potomac in this paper in the same sense as 

 that in which it was used in my earlier paper on The Potomac Formation," 

 especiall}- as given in the classification at the bottom of page 375, and in all 

 the subsequent tables and discussions. This is in the main the Potomac 

 formation as it occurs in Virginia, but also includes all beds of the same age 

 occurring in other States. It excludes from the Potomac formation, as I 

 have used the term, only those higher l^eds in which the flora is mainly 

 dicotyledonous, which are also veiy extensive and have yielded a rich 

 flora, and which, together with all other beds of practically the same age 

 (Tuscaloosa, Cheyenne sandstone, etc.), will fonn the subject of the next 

 or third paper of this series. 



HISTORICAL REVIEW. 



Although the object of this series of papers is pruiiarily to treat the 

 floras of the several formations considered, there is always a long period 

 during which the geological and lithologieal relations chiefly atrtract 

 attention, with only occasional reference to the more striking paleonto- 

 logical phenomena. These early groupings are of especial interest from 

 the historical point of view, and an account of them is essential to a full 

 understanding of the nature of the formation. This is practically true 

 of the Older Potomac, and the account will be made as complete as the 

 data will pei'mit. 



The earliest reference that I find to rocks of this age is contained in 

 two papers by Mr. B. H. Latrobe, one of which dates back to the year 

 1799.* On page 442 of this paper he states in a footnote that the light- 

 house at Cape Henry "is a good solid building of Rappahannoc freestone." 



"Fifteenth Ann. Rep. U. S. Geol. Survey, 189.5, pp. 307-397. 



'' Memoir on tlie sand-liills of Cape Henry in Virginia, by B. Henry Latrobe: Trans. Am. I'bil. Soc, Vol. 

 IV, Philadelpliia, 1799, pp. 439-443. 



