FLORA OF OLDER POTOMAC FORMATION. 349 



2. Iron-ore clays (No. 22 in the illustrations) . Tliis subdivision consists of a series 

 of beds of fine gray and lead-colored clays containing several courses of carbonate 

 of iron in flattened masses and nodules, varying in size from a pound or two to half 

 a ton or more in weight. The color of these clays is due to carbonaceous matter 

 (pp. 41,42). 



He very early found a portion of a tooth of a saurian reptile in 

 an iron-ore bed near Bladensburg. This he sent to Dr. Christopher 

 Johnston, who regarded it as indicating a new genus and named this 

 genus Astrodon." This tooth was more fully described later by Prof. 

 Joseph Leidy and the species named for Doctor Johnston, Astrodon 

 Johyistoni. '' It is barely referred to in Mr. Tyson's report on page 42 

 as coming from his No. 22, which is the older bed and includes the iron- 

 ore cla^^s. From this bed were also obtained the only plant remains, 

 consisting of "a new genus of Cycas of large dimensions," "silicified 

 coniferous wood," and "lignites (coniferous)." 



In his second report, published in 1862, Mr. Tyson discusses these 

 iron ores in Chapter VI, and on page 54 says : 



On page 42 of the first report I took occasion to refer briefly to what I called 

 iron-ore clays, numbered 22 in the table of formations. At that period it was believed 

 that this formation should be placed with the formations of the Cretaceous 'period in 

 geolog3^ Since then, however, I am disposed to place it at least as low as the oolitic 

 period. M. Agassiz, to whom I exliibited a photograph of the fossil Cycas (noticed 

 in the first report, and of which I have discovered several specimens in this forma- 

 tion) , fully agrees with me in this regard. 



These clays possess an especial interest to us from the existence therein of large 

 deposits of iron ores, from which it is believed the first pig iron was made in this 

 State. The quality of the metal is very superior. 



This ore, which is known as carhonate of iron, has a local name of Jione ore, from 

 its resembling a hone in its texture and color. It usually contains from 32 to 40 per 

 cent of iron. It exists in flattened nodules, varying in size from the weight of a 

 few pounds to one hundred or more, which are embedded in the iron-ore clays of 

 formation No. 22. 



These clays range nearly parallel with the lines of railroad from Washington, 

 via Baltimore and Havre de Grace, nearly to Elkton, and do not extend either into 

 Virginia or into Delaware. They constitute a formation peculiar to Maryland. 



« Am. Journ. Dental Science, New Series, Vol. EX, Philadelphia, July, 1859, p. 341. 

 i Smithsonian Contributions to Knowledge, No. 192, Vol. XIV, Article VI, 1865, p. 102, pi. xiii, figs. 20-23; 

 pi. XX, fig. 10. 



