FLORA OF THE DISTRICT OF COLUMBIA. 17 



Branch at Riggs Mill (PI. 3), where the change is most strongly 

 marked, and Paint Branch near Paint Branch School (PI. 1). 

 Characteristic Coastal Plain views are shown also in plates 10 and 13. 



The geological formations of the Piedmont Plateau are chiefly 

 crystalline gneisses of early periods, among which may be mentioned 

 the Carolina gneiss, granite gneiss, diorite, biotite, granite, and basic 

 igneous rocks, including gabbro, meta-gabbro, and serpentine. 



The Coastal Plain is occupied by unconsolidated gravels, sands, 

 and clays deposited during successive advances of the sea in Creta- 

 ceous, Tertiary, and Quaternary periods. Gravels also occur to 

 some extent as cappings on the hills along the edge of the Piedmont. 

 Bogs are frequent on the Coastal Plain and rare on the Piedmont. 

 Lime is often present in notable amount in the soils of the Piedmont 

 through concentration by means of accumulations of leaf mold, 

 although there are no highly calcareous rocks in the vicinity. Acid 

 soils are predominant on the Coastal Plain. The relation of soils 

 to the floras of the two regions is illustrated by the distribution of 

 the lily family and its aUies. Those species limited to calcareous 

 soils and found chiefly on the Piedmont Plateau are Allium tricoccum, 

 Vagnera stellata, MelantTiium latifolium, and Trillium sessile. Those 

 limited to noncalcareous or acid soils and found chiefly on the Coastal 

 Plain are Aletris farinosa, Tofieldia racemosa, Melanihium virginicum, 

 and Stenanthium gramineum. The other species of the Liliales grow 

 throughout the region and apparently have no marked soil preference. 



The floras of the two physiographic areas are noticeably different, 

 though there are many species common to the two regions. In the 

 Piedmont region there is a marked intrusion of mountain and north- 

 ern plants, these being found especially along the steep banks of the 

 Potomac and of the ravines leading into it, and along Rock Creek and 

 its western tributaries. The uplands, when not under cultivation, 

 for the most part represent land previously cultivated and now 

 covered with second growth forest or waste fields. The original 

 flora was less characteristically mountain than that which occupies the 

 ravines. 



The Potomac Valley region is easily accessible and offers a most 

 convenient and dehghtful field for a study of the spring flora. One of 

 the three troUey lines extending in this direction terminates at Cabin 

 John, from which the pedestrian may explore the left bank of the 

 Potomac and the ravines which enter the main valley at intervals 

 above. Though the residential district is extending up the Potomac, 

 there are still many ravines and banks between Georgetown and 

 Cabin John of interest to the collector. These may be reached from 

 the trolley line or from the Conduit Road, which follows this line and 

 continues to Great Falls. The other two of the three trolley hues 

 69289—19 2 



