FLORA OF OLDER POTOMAC FORMATION. 343 



This shows that the freestone quarries on and near the Rappahannock 

 River had long been worked at that time. The other paper, pubhshed 

 ten years later," is devoted to "the freestone quarries on the Potomac 

 and Rappahannoc, from the former of which the freestone employed in 

 the public buildings of the United States at Washington is obtained" 

 (p. 284). He gives a good description of the freestone rock, including 

 that of the clay nodules so characteristic of it. On page 287 he says: 



Wood, from trunks and branches of trees of large size to small twigs, either 

 entii-ely carbonated or the wood carbonated and the bark in a fibrous state, so as to 

 have the appearance of a net, and a considerable degree of tenacity; or the bark 

 fibrous and the wood in a state quite friable; or the wood replaced by pyrites which 

 effloresce in the air; or in cavities the sides of which have the impression of branches 

 in minute ramification and are lined with a pellucid crust, probably calcareotis spar. 

 Tins latter evidence of the admixture of wood is to be found chiefly near Fredericks- 

 burg. 



On July 15, 1823, Mr. John Finch read a paper before the Academy 

 of Natural Sciences of Philadelphia " in which he, classed all the beds of 

 the coastal plain as Tertiary and compared them with those of Europe. 

 On page 39 of this paper he says: "At Washington, under the mass of 

 diluvian gravel of which the higher part of the Capitol hiU is composed, 

 there is a stratum of clay which contains many organic remains. Trunks 

 and branches of trees are found at a distance of fifty-four feet from the 

 surface." It is probable that these remains were in the Potomac forma- 

 tion, although they may have been in the overlying Columbia formation, 

 in which such objects have been found within the city of Washington. 



In 1829 Messrs. Morton and Vanuxem published a paper of a very 

 general character,^ but their "Secondary formation" evidently includes 

 the whole of the Potomac formation and also the marls of New Jersey. 

 In the following statement they exactly describe the conditions under 

 which the wood and lignite of the Potomac formation occur: 



In many of the States there is a bed of clay (No. 2 of the diagram) containing 

 lignite or charred wood, with pyrites, amber, etc., which is no doubt represented in 



n An account of the freestone quarries on the Potomac and Rappahannoc rivers, by B. H. Latrolje: Trans. 

 Am. Phil. Soc, Vol. VI, Pt. II, 1809, pp. 283-293. 



b Geological essay on the Tertiary formations in America, by John Finch : Am. Journ. Sci., ongmal series, 



Vol VII, 1824, pp. 31-43. 



c Geolo.'ical observations on the Secondary, Tertiary, and alluvial formations of the Atlantic coast of the 

 United States of America, arranged from the notes of Lardner Vanuxem, by S. G. Morton, M. D. : Journ. Acad. 

 Nat. Sci. Philadelphia, Vol. VI, Pt. I, 1829, pp. 59-71. 



