344 MESOZOIC FLORAS OF UNITED STATES. 



many places by beds of sand containing woody fiber replaced by siliceous matter, 

 for in all cases where wood is enveloped by clay, which admits with difficulty the 

 percolation of water, the mass is found in a black, charred state; but, on the con- 

 trary, when deposited La a matrix which admits the infiltration of water, such as 

 sand, soil, or loam, the wood appears in the replaced or petrified state. 



Dr. Edward Hitchcock, in his early report, pubhshed in 1833, had 

 of course only to deal with the northern extension of the Potomac forma- 

 tion, which contains none of the older beds, but he was aware of the great 

 extent of this formation, as evidenced by the following remark: 



The patches of this formation that have been described in Massachusetts are 

 doubtless only the remnants of a vast extent of these strata, extending at least from 

 Cape Cod to the borders of the Gulf of Mexico, and how far eastward, where the 

 Atlantic now rolls, we can form no probable opinion, though there is some reason for 

 supposing that they once even reached Europe, along whose shores similar strata are 

 found at present/' 



Messrs. Clemson and Taylor commenced their extended investiga- 

 tions into the geology and mineral resources of Virginia at about this 

 time and published their first papers in 1835." Mr. Clemson concludes 

 his paper with a fairly full description of the Potomac beds in the vicinity 

 of Fredericksburg, including that of "Alum Rock," a mile south of that 

 place. He also refers to the lignites and fossil wood found in that region, 

 and speaks of "a blue argillaceous bed six inches to a foot in thickness, 

 which divides easily and displays to view fine impressions of plants" 

 (pp. 312-313). This appears to be the earliest mention of plant impres- 

 sions in the Older Potomac formation, and may allude to the same locality 

 where Professors Uhler and Fontaine later obtained so large an amount 

 of material of this class. 



Mr. Taylor, in the paper"' that immediately follows this, devotes six 

 pages (320-325) and one folded plate (pi. xix) to the description and 

 illustration of the plants from this locality. He states that his "attention 

 was first directed to these plants by Mr. F. Shepherd, who at our request 



« Report on the Geology, Mineralogy, Botany, and Zoology of Massachusetts, by Edward Hitchcock, 

 Amherst, 1833, pp. 201-202. 



b Notice of a geological examination of the country between Fredericksburg and Winchester, in Virginia, 

 including the gold region, by Thomas G. Clemson: Trans. Geol. Soc. Penna., Vol. I, Pt. II, Philadelphia, 183.5, 

 pp. 298-313, pi. xvii. 



c Review of geological phenomena, and the deductions derivable therefrom, in two hundred and fifty miles 

 of sections in parts of Virginia and Maryland; also notice of certain fossil acotyledonous plants in the Secondary 

 strata of Fredericksburg, by Richard C. Taylor: Ibid., pp. 314-325, pis. xviii-xix. 



