CONCEALMENT OF GEOLOGIC STRUCTURE. 305 



and banding characteristic of gneisses. Another effect of this profound 

 metaraorphism has been to obliterate the originally sharp contact lines be- 

 tween diverse rock-raasses. The chemical changes have brought about an 

 interchange of material along such lines, so that rocks once very distinct 

 now appear to grade imperceptibly into one another. 



Superficial Decay. — This is another cause which renders both areal map- 

 ping and the deciphering ef the complicated structure difficult in the Pied- 

 mont region. In Maryland, as in all districts south of the glacial line, the 

 rocks are in a state of incipient, if not advanced, decay at or near the sur- 

 face. This tends to render similar rock-types indistinguishable, as in the 

 case of many gneisses and granites which are composed of about the same 

 constituent minerals. The same cause often develops a most misleading 

 cleavage in many massive rocks. This is the case particularly in the large 

 Sykesville granite area, a part of which was first mapped as gneiss until a 

 deep opening into it revealed inclusions of other rocks, which placed its in- 

 trusive origin beyond a doubt. 



This decay is also frequently the means of overturning the steep dip of the 

 beds. This may be seen in many quarries, where the lower dips are in the 

 opposite direction from those within ten feet of the surface. 



Cultivation. — This tends, of course, to cover up rock exposures, often just 

 where they are most necessary for the deciphering of the structure. 



Petrography of the Piedmont Region in Maryland. 



The marked contrast between the semi-crystalline and highly crystalline 

 rocks of the western and eastern portions of the Maryland Piedmont region 

 was recognized by Philip Tyson, and roughly indicated on his geological 

 map of the state, published in I860.* The line of boundary between the 

 two areas has already been approximately traced by the writer. Their 

 essential character will be clearly seen by a brief description of the various 

 rock-types thus far identified and studied in each. 



ROCKS OF THE WESTERN OR SEMI-CRYSTALLINE AREA. 



Sedimentary Series: Phyllite. — Under this designation may most con- 

 veniently be included the semi-crystalline slates and finely fissile schists 

 which compose so large a portion of the western Piedmont area in Mary- 

 land. They are capable of subdivision into a great number of varieties, all 

 of which, however, fall clearly within the general characterization of phyllite, 

 given by Creduer.f They are beyond doubt argillaceous sediments which 

 have undergone a greater or less amount of mechanical (cleavage) and 



* First Annual Report of the State Agricultural Chemist to the House of Delegates of Maryland : 

 Annapolis, 1860. 

 fElemente der Geologie: 5th edition, 1883, p. 111. 



