98 Bulletin i. Biological Society of Washington, 1918. 



ceoiis gneiss may be seen. On the north side of the Occo- 

 quan River, opposite the village of Oecoqnan, the high hills 

 that overlook the river seem to be composed wholly of mica- 

 schist. 



"From the village of Occoquan due east down the river 

 to Woodbridge, where the railroad crosses the river, the 

 distance is 2 miles (p. 113). The station is on the opposite 

 side of the river from the site of the former village of Col- 

 chester, where the Colchester road crosses the river. At 

 Woodbridge the railroad approaches the bridge by a long 

 cut. In this cut the Potomac formation may be seen resting 

 on pre-Cambrian slate similar to that which, at Dumfries 

 and northward, forms its western boundary. The bottom of 

 the Potomac formation is here about 40 feet above the river. 

 The formation, in the interval of 2 miles between this sta- 

 tion and the village of Occoquan, has declined toward the 

 east certainly as nuich as 160 feet" (p. 114). 



Growing among the Piedmont rocks immediately above 

 Occoquan village is the most extensive colony of Rhodo- 

 dendron })ia.ri7uuiii known in our region. Some of the north- 

 ern exposures of the main ravine and its tributaries have 

 patches of the plant covering acres. Wintergreen is abund- 

 ant near Occoquan and the Pyrola, Lycopodium and even 

 white pine that characterize our most marked Piedmont 

 woods have been found. Hemlock is especially common, 

 I^robably more of it growing here than in all other places 

 together within the same distance of Washington. It fol- 

 lows the Piedmont rocks to their very limit, and due to the 

 character of the exposure (as described above) it comes 

 about that hemlocks can be found with their roots bathed 

 by the waters of a tidal marsh. This is a remarkable occur- 

 rence for our latitude. 



Occoquan is further of interest as a station for the hairy 

 fern, Cheilantlies lanosa, for the occurrence of Filago ger- 

 manica, and for the persistence in its environs of the wild 

 turkey. 



Returning to localities nearer Washington some account 

 will be given of Glencarlyn and Falls Church of the Rock 

 Creek Region, Takoma Park, and finally of the chief spots of 

 interest along the upper Potomac. 



