FhOUX OF OLOEK POTOMAC FORMATION. 349 



'J. Iron-on^ clays (No. l'l' in tlic illustrations). This snl)(li\isi(in consists of a sci-ies 

 of Ix'ds of line <;fa\' and Icad-rolorcil clays containini: scxcral c-ourscs of carKonatt' 

 of iron in llallcncd masses and nodules, varyintj in si/.c from a pound or twt) to half 

 11 ton or more in \\eii;hi. The color of tiiese clays is due to carbonaceous matter 

 (pp. 41, 42). 



lie vei-\' (>;(i-l\' fotiiid :i poftioii of a looth of a saiifiaii toptilc in 

 an ifoii-ofo hcd near Bladciishiift;-. This lie .sent 1o Dr. Clirislopher 

 Johii.stoii. who i'eg;anle(l it as iiulicatiiiii a now iieinis and named this 

 genus Astrodon." This tooth was n^ore ftilly deseril)ed latei- hy Pfof. 

 Jo.seph Leidv and the species named for Dortoi- Johnston, Astrodon 

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

 as coming from his \o. 22, which is the older l)ed and includes the iron- 

 or(> clavs. I'rom this bed were also obtained the onl.\' plant rcMiiains, 

 consisting of "a new genus of (Vcas of large dimensions," "silicified 

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



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

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



On page 42 of the tirst report I took occasion to refer brielly to what 1 called 

 iron-ore rl(nj.% numbered 22 in the table of formations. At that period it was believed 

 that this formation should be placed with the formations of the Crctiurou.s period in 

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

 period. M. Agassiz. to whom 1 exhibited a. |)hotogra|>li of the fossil C'ycas (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 carhonatf of iron, has a local luime of hone ore, from 

 its resenibling 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 hundr(>d or more, which are embedded in the iion-oie clays of 

 formation No. 22. 



These clays range nearly parallel with the lines of railroad fnjm Wasliington, 

 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. IX, Philadelphia, July, 18,59, p. 341. 

 '' Smithsonian Contributions to Knowledge, No. 192, Vol. XIV, Article VI, 186.5, p. 1(12, pi. .\iii, tigs. 2(>-23; 

 pi. XX, fig. 10. 



