276 Sinnott and Bartlctt — Coniferous Woods. 



Art. XVIII. — Conifer ons Woods of the Potomac Formation / 

 by E. W. Sinnott and H. H. Baktlett. 



The Potomac formation is of particular interest to botanists 

 because of the rich flora which it contains. Its leaf impres- 

 sions have become known through the labors of Fontaine,* 

 Ward,+ and Berry,;}; but the fossil woods have attracted far 

 less attention. The only important contribution to our knowl- 

 edge of the latter was made in 1889 by Knowlton,§ who 

 described from silicified material four species of Cupressinoxy- 

 lon ( C. pulcheUum, C. 3fcGeei, C. Wardi, and C. columbi- 

 anumj. The Potomac formation contains far more lignite 

 than silicified wood, but no one has thus far studied it with 

 satisfactory results. After examining many specimens Knowl- 

 ton wrote : " From the abundance of the genus Cupressinoxy- 

 lon in the Potomac formation, as shown by the silicified 

 examples, it is probable that most of the lignite may be also of 

 this genus, particularly as there is in many cases a marked 

 resemblance, so far as I am able to interpret the distorted 

 structure, between it and some of the species described from 

 silicified specimens. This is, however, little more than con- 

 jecture." Only recently Berry has said : " Both lignite and 

 petrified wood, either silicified or ferruginized, are common at 

 most horizons in the Potomac beds, but very little effort has 

 been devoted to their collection or study. The lignites as far 

 as they have been examined are coniferous but so much dis- 

 torted by pressure and by decay before fossilization as to be 

 of but little scientific value. The silicified materials, while in 

 a somewhat better state of preservation, leave much to be 

 desired. Of a large number of such sections in the hands of 

 the writer only a very few are sufficiently well preserved to be 

 determinable. The ease with which fragments of silicified 

 wood may be reworked from formation to formation also min- 

 imizes their stratigraphic and phylohistorical value." 



One of the writers has recently seized an opportunity to 

 secure abundant material of lignite and charcoal from the 

 Patuxent horizon of the Potomac formation at two localities 

 in the District of Columbia. A deep and extensive excavation 

 for the new Central High School of Washington at 11th Street 



* Fontaine, W. M., The Potomac or younger Mesozoic Flora, Mon. U. S. 

 Geol. Surv., xv, 1889. 



fWard, L. F., The Potomac formation, Fifteenth Ann. Eep. U. S. Geol. 

 Surv., pp. 307-397, 1895. 



\ Berry, E. W., Several papers, summarized and referred to in Maryland 

 Geol. Surv., vol. iv, Lower Cretaceous, Baltimore, 1911. 



§Knowlton, F. H., Fossil wood and lignite of the Potomac formation, 

 Bull. U. S. Geol. Surv. No. 56, 1S89. 



