Natural History of District of Columbia— McAtec !>7 



southern robber fly Erax nemoralis. With Marshall Hall is 

 shared the little lace bug, Leptoypha costata. 



At Marshall Hall have been found such austral plants as 

 bald cypress (also at Bowie), Polypremum procumbens 69 

 and Pluchea camphorata, and rather surprisingly such high- 

 land ferns as Athyrium pycnocarpon, Cystopteris fragilis, 

 and Camptosorus, and Trautvetteria which are more char- 

 acteristic of Piedmont areas. A paper based on investiga- 

 tions at Marshall Hall, and which altogether gives a rather 

 full discussion of the local fauna and flora, is Dr. S. D. 

 Judd's "Birds of a Maryland Farm, A Local Study of Eco- 

 nomic Ornithology" (Bui. No. 17, U. S. Biological Survey, 

 1902, 116 pp., 17 Pis., 41 Figs.). 



One of the most interesting downstream localities is Occo- 

 quan, Va., and perhaps the chief reason for its attractiveness 

 is the excellent opportunity for study of the line of contact 

 of the Piedmont and Coastal Plain formations and their 

 accompanying biotas. The old village of Occoquan is one 

 of the numerous settlements that were made on the Fall 

 Line, and in this case the natural advantages of a good 

 bridge site, power from rapids, and deep water for shipping 

 are admirably concentrated. 



With regard to geological conditions at Occoquan, Fon- 

 taine 70 says: "In the top of the hill overlooking the river 

 and in the village a thin remnant of the Potomac [a Coastal 

 Plain formation] may be seen. It is composed of gray, 

 argillaceous sand, variegated with yellowish-brown and red- 

 dish colors. In this vicinity the Potomac has been almost 

 wholly removed by erosion and is visible only on the highest 

 points. The depth to which the depressions have been cut 

 since the deposition of this formation indicates for it a 

 great age. In the immediate vicinity of the village of Occo- 

 quan the thickness of fully 200 feet of pre-Cambrian rock 

 is exposed under the Potomac, being laid bare by erosion in 

 the post-Potomac times. These pre-Cambrian rocks are no 

 longer wholly slates, but in the lower portions rather coarse- 

 grained and highly crystalline mica-schist and highly mica- 



•Has been collected at Benning also. 



70 Fontaine, W. M. The Potomac Formation in Virginia. Bui. 145, U. S. 

 Geol. Survey, 1896, pp. 113-4. 



