Mathews — Structure of the Piedmont Plateau. 147 



of the phyllites and mica-schists the original material has been 

 changed to muscovite, chlorite, and quartz with accessory min- 

 erals such as garnet, staurolite, cyanite, etc. 



Structure of the Maryland Piedmont. 



The four types of metamorphosed rocks with their accom- 

 panying igneous types unite in the Piedmont area of Maryland 

 to form a complex geological mass in which the structure of 

 the region is very greatly obscured by the metamorphism, 

 secondary schistosity, and recrystallization. The problem is 

 still further complicated by the presence of several series of 

 minor folds which obscure the larger structural features of the 

 region. The centering of attention on these minor structural 

 features rather than on the broader elements of structure have 

 led previous students of the area to divergent interpretation, 

 or despair at ever reaching a true solution. 



Previous Interpretations. — The Piedmont Plateau of Mary- 

 land has been studied by many investigators and the interpre- 

 tations which they have given to the area may be classified, as 

 suggested by Williams, into three categories, involving differ- 

 ences in age in the sedimentaries of the eastern and western 

 portions of the area or differences in the structural elements 

 which have brought the rocks to their present position. The 

 three hypotheses as outlined by Professor Williams'* are: 



"1. That the rocks of both the eastern and western areas are 

 of the same age, and that they have been bent into a broad 

 syncline whose flanks are so sharply folded, faulted and thrust 

 as to simulate the fan-structure observed in high mountain 

 chains ; and that the eastern flank of this synclinal or fan was 

 much more highly metamorphosed than the western both by 

 more intense dynamic action and by intrusion of a great amount 

 of eruptive material. 



2. That the more highly crystalline eastern area is greatly 

 older than the western schists, and served as a rigid buttress 

 against which these were thrust and folded. 



3. That the eastern area is composed of rocks far more 

 ancient than the western, which extend out under these, form- 

 ing the floor upon which they were deposited ; and that although 

 already much folded and metamorphosed, this crystalline floor 

 underwent at least one more folding after the schists had been 

 laid down, carrying these with it and involving them in a con- 

 siderable but not an extreme amount of disturbance and meta- 

 morphism." 



The first of these hypotheses was that held by Tyson, who 

 was State Agricultural Chemist from 1853 to 1862, and who 

 published the first complete geological map of Maryland in 



* Bull. Geol. Soc. Amer., vol. ii, p. 315. 



