1920.] NATURAL SCIENCES OF PHILADELPHIA, 91 



in spite of a rather high content of alkah and alkahne earth metals, — 

 sodium, calcium, magnesium,— gives mostly subacid to minimacid 

 soils. Serpentine behaves similarly, and subacid or even mediacid 

 soils often develop, in spite of the magnesimn content. The red 

 shale formation of Triassic age, which covers a wide area in this part 

 of Pennsylvania, yields subacid soils in many places, but since some 

 of its strata contain considerable calcium carbonate, minimacid, 

 neutral, and even minimalkaline soils are at times present upon it. 

 The abundance of Ericaceae on these several formations has been 

 found to follow closely the development of soils of subacid or 

 mediacid reaction. 



Hills Northwest of Washington, D. C. 



On coming further south, the rocks are mostly more fully decom- 

 posed than they are in Pennsylvania, and the alkali metals are more 

 extensively leached out, so that hills are often covered with dense 

 clays, showing but little similarity to the original rocks. Subacid 

 soils have been found to be dominant in such situations, and Erica- 

 ceae accordingly fairly common. Along the Potomac Valley north- 

 west of Washington, D. C, the clays are considerably eroded, and 

 the solid rock exposed; but here a secondary factor influences the 

 soil reaction. The Potomac River and its tributaries flow for long 

 distances through limestone regions, and take up considerable 

 amounts of calcium bicarbonate. Not only does this make the 

 waters themselves minimalkaline, but the sands and muds deposited 

 at flood times also contain so much calcareous matter as to render 

 the soils there circumneutral in reaction. Accordingly the Erica- 

 ceae, which grow on the uplands, drop off markedlj^ as the level of 

 river deposition is approached, and circumneutral-soil plants are 

 common at the lower levels. 



Mountain Ridges of West Virginia. 



In the mountains of West Virginia, conditions are similar to 

 those in the corresponding regions of Pennsylvania. The Ericaceae 

 characteristic of the southern Appalachian Mountains reach their 

 northern limits there, but they have not been found to show on the 

 whole any differences in soil acidity from the Ericaceae which also 

 grow farther north. The upland peat of the great RJiododendron 

 thickets is mediacid to subacid in reaction, the plant roots usually 

 extending down into material of the lesser acidity. The reactions 

 of the soils of individual species are listed below. 



