158 Mathews — Structure of the Piedmont Plateau. 



area only relatively small bodies of serpentine and granite come 

 in contact with the younger gneisses and schists, and where such 

 is the case the intrusive body has been so small as to produce 

 no appreciable metamorphism. Moreover, if any contact 

 metamorphism occurred, it would now be practically unobserv- 

 able since the entire region has been more or less metamor- 

 phosed and contacts are almost entirely lacking in fresh rock. 

 "The sudden disappearance of the abundant eruptive rocks at 

 the edge of the western area," as inferred by Williams, is now 

 recognized to be a conclusion not wholly in accord with the 

 facts. Keith in his work in Frederick county found numerous 

 bodies of aporhyolite and metamorphosed acid volcanics and this 

 find has been corroborated and extended by the field work of the 

 writer. In this region of so-called semi-crystalline rocks the 

 eruptives found by Keith and others have been so metamor- 

 phosed into sericitic schists that they are scarcely distinguish- 

 able from the nearby phyllites except for the presence of small 

 quartz and feldspar phenocrysts. 



The points raised by Williams that the contact-between the 

 rocks of the eastern and western areas does not coincide with 

 the axis of the syncline and that the eruptives were much 

 metamorphosed at points where one would infer from the 

 structure but little metamorphism, involve the assumption of a 

 rather simple synclinal or fan-like structure which has already 

 been shown to be discordant with the facts as developed by 

 later and detailed investigations. 



From the foregoing paragraphs it may be readily seen that 

 the writer does not believe that the points raised by Williams 

 validly disposed of the contemporaneous age of the rocks in 

 the eastern and western portions of the Piedmont. Moreover, 

 the detailed work of Keith on the west and the reconnaissance 

 work by the Maryland Geological Survey on the north, point 

 to an infolding of the various rocks which ultimately may show 

 their similarity in age, when the detailed work has been com- 

 pleted. 



Conclusions. 



From the past ten years of work in the detailed mapping of 

 the Piedmont rocks of northern Maryland the author has been 

 led to the following conclusions as best in accord with the facts 

 in hand : 



1. The older rocks of the Piedmont consist of both sedimen- 

 tary and igneous types which since their formation have been 

 more or less metamorphosed. 



2. The metamorphosed sediments include banded micaceous 

 and hornblende gneisses of pre-Cambrian age ; a more or less 

 intermittent thin-bedded generally tourmaline-bearing quartz- 



