156 Mathews — Structure of the Piedmont Plateau. 



entirely lacking, and any contact metamorphism which they 

 may have imposed upon the rocks existing at the time of their 

 intrusion is now obscured by the subsequent metamorphism 

 which they and the associated country rock have undergone. 

 From the fact that parts of this igneous complex are intruded 

 into the rocks regarded as Silurian in age, this activity could 

 not have ceased prior to the later portion of the Silurian and 

 may have been much later. That the intrusions are not 

 younger than Paleozoic time seems to be an inference well 

 supported by the fact that they along with the sediments par- 

 take of the broad folding characteristic of the Appalachian 

 region to the westward, which was probably developed contem- 

 poraneously in the rocks of the eastern and western portions 

 of the state. Moreover, the sedimentaries of Jura-trias time 

 show little folding of this type, their deformation being due 

 almost entirely to very gentle folds and small faults of slight 

 throw. 



Relation of the Eastern and Western Districts of the Piedmont 

 The division of the Piedmont Plateau into an eastern divi- 

 sion composed of much metamorphosed, highly crystalline 

 rocks, gfnd a western division characterized by less metamor- 

 phosed, so-called semi-crystalline, rocks has long been recognized 

 but was first sharply emphasized by the late Professor Williams 

 in 1891. He regarded the eastern area as composed of rocks 

 far more ancient than those in the western district and that 

 they extended westward, forming a floor upon which the 

 younger phyllites were deposited. He also believed that the 

 eastern portion had already been much folded and metamor- 

 phosed before the phyllites had been laid down. As conclu- 

 sive against the identity of age of the semi-crystalline and non- 

 crystalline rocks he summarized the following points.* 



a. The structure is not really a synclinal, but a fan-like divergence of dip 

 from a central vertical axis, such as could not be produced by any synclinal 

 bending in a continuous series of similar beds. 



b. Any cause altering any part of an original series more than another 

 would not make an abrupt contact, such as we find between the semi-crystalline 

 and highly crystalline rocks of Maryland, but a gradual transition. 



c. Any cause altering one flank of a synclinal more than the other 

 would make the contact between th& two kinds of rock and the axis of the 

 synclinal coincide, as is not the case in Maryland. 



d. The eruptive rocks of the eastern area are found in many places in close 

 proximity to the slates or schists, without having effected their alteration ; 

 hence they are either not the cause of metamorphism, or the} 7 are themselves 

 older than the semi-crystalline rocks ; and, moreover, the sudden disappear- 

 ance of the abundant eruptive rocks at the edge of the western area is itself 

 a strong reason for supposing that it is of later age. 



e. We cannot suppose that excessive dynamic action was the cause of the 

 metamorphism, because where we should expect the folding force to have 

 acted equally we find the hardest rocks (eruptives) much more altered, foli- 

 ated, and disturbed than the soft argillites. 



*Bull. Geol. Soc. Amer., vol. ii, 1891, p. 316. 



