146 Mathews — Structure of the Piedmont Plateau. 



and larger intrusive bodies were thrust into their present posi" 

 tion is not known, but the views most generally held at the 

 present time indicate that the intrusion took place in post- 

 Silurian time prior to the earth movements which produced the 

 Appalachian Mountains. Earlier investigators and some at the 

 present time express the opinion that this igneous activity was 

 contemporaneous with the formation of the earliest rocks. 



Metamorphism.— The older rocks of the Piedmont, as recog- 

 nized by all investigators, have suffered more or less recrys- 

 tallization and textural modification since their formation. This 

 metamorphism has not been uniformly distributed over the 

 entire region but, as emphasized by Williams, is much accentu- 

 ated in the eastern portion of the Maryland area, where the 

 rocks are thoroughly recrystallized and often lack in great meas- 

 ure their original texture. The original muds and sands of the 

 sedimentaries have been changed to micaceous schists, gneisses, 

 and quartzites and the various igneous rocks have been greatly 

 modified in texture and occasionally in mineralogical composi- 

 tion. The textural change which is most evident is a marked 

 development of schistosity which is to be noticed in all of the 

 rock tyrfes already described. The change from massive to 

 schistose rocks has not been uniform over the entire district or 

 even over the more metamorphosed eastern section, but seems to 

 be locally accentuated along lines which probably indicate zones 

 of greater dynamic action. 



The schistosity developed in the rocks of the Piedmont par- 

 takes of the general northeast-southwest trend of the province 

 and varies in dip sometimes to the eastward and sometimes to 

 the westward. It is present in both the sedimentary and igneous 

 rocks. In the latter, it is sometimes so strongly developed 

 that the resulting rocks in small areas present the appearance 

 of metamorphosed sediments, although one may find all grada- 

 tions between the unaltered massive types and the equivalent 

 fissile schists. In the sedimentary rocks the schistosity is devel- 

 oped to a degree which greatly obscures the original bedding 

 and oftentimes renders the determination of bedding planes 

 impossible. 



The development of schistosity is accompanied by a recrys- 

 tallization of the affected rocks, which may simply result in a 

 new development of the mineral species found in the original 

 rock or in a molecular rearrangement producing many new 

 minerals. Thus the gneisses are composed of recrystallized 

 quartz, feldspar, and mica material, while the feldspars of 

 some of the granites, the meta-rhyolites, gabbros, and diorites 

 have been changed to epidote and the pyroxenes to fibrous or 

 compact hornblende. The new minerals usually lie with their 

 longer axes parallel to the planes of schistosity. In the case 



