200 THE PLANT LIFE OF MARYLAND 



Geological Survey, where one of the most striking characteristics of 

 the district has heen brought out, namely the close correspondence 

 between the distribution of the soils and that of the geological 

 formations. The several outcrops lie parallel to the general length 

 of the district, so that it is possible, in terms of the conditions in 

 the above-named counties to interpret the soils of the other counties 

 in the technical nomenclature used upon the Soil maps. 



The soils which cover the greatest area are the Cecil loam and 

 the Cecil mica loam, derived from granite, gneiss and diorite of 

 Archean or Paleozoic age. While distinct from the standpoint of 

 the soil physicist, these two types, which differ only in the pres- 

 ence of mica fragments in the latter, are identical so far as their 

 texture bears any relation to the amount and availability of soil 

 water. The vegetation of the loams is that which is most typical 

 of the Lower Midland District, not only because these soils are the 

 most extensive in area, but because of the variety and richness of 

 the arboreal and herbaceous flora, favored by the optimum soil con- 

 ditions which they present. Second in extent are the areas of Cecil 

 clay, derived from the weathering of gabbro, of which the largest 

 lie directly west of Baltimore and in Harford County. There is 

 not a very great difference between the Clay and loam vegetation, 

 the former differing from the latter chiefly in the rarity and ab- 

 sence of certain plants which are more abundant on sands and other 

 lighter soils. Relatively small areas are underlaid by serpentine, 

 which either weathers to the Conowingo clay or presents the thin- 

 soiled areas known as Serpentine Barrens. These occur in every 

 county along a narrow line extending from a point where the Sus- 

 quehanna enters the state to the banks of the Potomac just above the 

 Great Falls. Their vegetation is well known as being very distinct, 

 both through the abundance of species not common elsewhere and 

 the absence of a large part of the flora of the surrounding soils. 

 In Baltimore County there is an area in which marble forms the 

 underlying rock, extending from Towson to Cockeysville, with irreg- 

 ular arms to the east and west. This interesting area is entirely 

 under cultivation wherever it is not covered by Cretaceous or 

 Pleistocene deposits, and the natural vegetation of its agriculturally 

 rich clay soil is therefore unknown. Tlio cretaceous deposits which 



