304 G. H. WILLIAMS — STRUCTURE OF THE PIEDMONT PLATEAU. 



limestone valleys to cut their way for long distances through steep ridges of 

 gneiss or schist. Nor do they here follow the line of least resistance, but 

 cross the strike as often as they follow it. 



Geological Subdivisions. — Geologically, the Piedmont region in Maryland 

 is divided into an eastern and a western section with even greater sharpness 

 than it is divided topographically. On the one side, disappearing beneath 

 the coastal deposits, occur the holocrystalline rocks which, whatever be their 

 origin, now retain no certain evidence of clastic structure ; while on the west 

 all the schists, limestones and sandstones are more or less clearly of sedi- 

 mentary origin. The line separating these two divisions of the Piedmont 

 plateau, which we shall hereafter designate as the semi- crystalline (western) 

 and crystalline (eastern) areas, is not coincident with the crest of Parr's 

 ridge, but lies on its eastern flank. Commencing in the south near Great 

 Falls on the Potomac, it passes slightly west of Rockville and of Hood's 

 Mills, then through Finksburg on the Western Maryland railroad, and 

 thence by a north-northeastward course to the Pennsylvania line. Further 

 eastward there is a large area of the semi-crystalline schists in Harford 

 county, surrounding the Peach Bottom and Delta roofing slates. These 

 appear to be infolded in the gneisses, and are possibly connected with the 

 main area by a narrow tongue passing the Northern Central railroad near 

 Whitehall. A similar infold of slates also occurs further southward, near 

 Occoquan and Dumfries in Virginia. The lines of demarcation between 

 these infolded areas and the surrounding gneisses are not less sharp than 

 those separating the main crystalline and semi-crystalline areas themselves. 



Sources of Difficulty in deciphering the Geology of the 

 Piedmont Area. 



Primary Sources. — Before turning to a special consideration of the petrog- 

 raphy and structure of Maryland's crystalline and semi-crystalline areas, 

 it may be well to briefly specify the chief sources of difficulty encountered 

 in attempting to decipher the complex geology of the region. These are in 

 the main three, viz., metamorphism, superficial decay, and extensive cultiva- 

 tion of the soil, all of which tend to obscure original contacts and structures, 

 however sharp and well defined they may once have been. 



Metaynorphism. — This difficulty applies chiefly to the eastern or more crys- 

 talline area, where it has been carried so far as not merely to disguise the 

 original structure of the rocks, but, in many cases, even to render it uncer- 

 tain whether they are sedimentary or eruptive. Some of the rocks which 

 were once probably sedimentary deposits have been wholly recrystallized 

 and their new mineral constituents united in a quite massive aggregate ; 

 while other areas of undoubted eruptive origin have assumed the foliation 



