Mathews — Structure of the Piedmont Plateau. 151 



the major folds, as determined by tracing individual beds 

 with, their sinuosities across the larger exposures, are much 

 flatter than those of the minor folds, usually ranging in value 

 from 5° to 40° with an average of between 25° and 30°. In 

 many instances it is not possible to determine this major dip, 

 but the areal distribution of the rocks is generally found to be 

 in harmony with such observations as can be made. 



Overturned folds are occasionally encountered, the best 

 example so far studied occurring just west of the Baltimore- 

 Harford county boundary line, about five to ten miles south of 

 the phyllite boundary. The structure here is well marked by 

 the occurrence of a quartzite, an intermittent marble, and the 

 mica-gneisses surrounding a core of banded mica- and horn- 

 blende-gneisses. Contrary to the usual interpretation, this large 

 fold is overturned to the southeast, causing all the dips to slope 

 to the west. 



Unconformities apparently occur in the crystalline rocks 

 of the Piedmont, especially at the base of the quartzite and 

 the top of the marble, but it is very difficult to determine 

 whether or not the latter is due to a change in the character 

 of the original limestone which, when highly argillaceous, is 

 metamorphosed into a calcareous gneiss or mica-schist which 

 is practically indistinguishable by field observation from the 

 adjoining rocks. The points where this unconformity has been 

 supposed lie near the limits of the area in which marble occurs, 

 and it seems highly probable that the conditions of the forma- 

 tion of a limestone became less favorable around the limits of 

 the area, and that through gradation the limestones pass off 

 into rocks which, when metamorphosed, are indistinguishable 

 from the overlying mica-schists. 



Geological Sequence. — The field work conducted by the 

 author and his associates on the Maryland Geological Survey 

 indicates that in the Maryland Piedmont we have the follow- 

 ing series of metamorphosed rocks which were apparently of 

 sedimentary origin. 



Peach Bottom slates ) a-t • /q\ 



Cardiff quartzite [ _ Sllumn (? ) 



Wissahickon phyllite, mica-schist ) ^ i 



and mica gneiss [ Ordovician 



Cockeysville marble Cambro-Ordovician 



Chickies quartzite or \ x ~ , . 



Setters quartz schist f Lower Cambrian 



Baltimore gneiss Pre-Cambrian 



The determination of the age of these formations by fossils 

 is impossible at the present time in Maryland, and the age 

 assigned the various beds is therefore regarded as somewhajb 



