316 G. H. WILLIAMS STRUCTURE OF THE PIEDMONT PLATEAU. 



of age for the semi-crystalline and holocrystalline rocks we may summarize 

 the following points : 



a. The structure is not really a synclinal, but a fan-like divergence of dip 

 from a central vertical axis, such as could not be produced by any synclinal 

 bending in a continuous series of similar beds. 



h. Any cause altering any part of an original series more than another 

 would not make an abrupt contact, such as we find between the semi-crystal- 

 line and highly crystalline rocks of Maryland, but a gradual transition. 



c. Any cause altering one flank of a synclinal more than the other would 

 make the contact between the two kinds of rock and the axis of the syn- 

 clinal coincide, as is not the case in Maryland (see map, plate 12). 



d. The eruptive rocks of the eastern area are found in many places in 

 close proximity to the slates or schists, without having effected their alter- 

 ation ; hence they are either not the cause of metamorphism, or they are 

 themselves older than the semi-crystalline rocks ; and, moreover, the sudden 

 disappearance of the abundant eruptive rocks at the edge of the western 

 area is itself a strong reason for supposing that it is of later age. 



e. We cannot suppose that excessive dynamic action was the cause of the 

 metamorphism, because where we should expect the folding force to have 

 acted equally we find the hardest rocks (eruptives) much more altered, 

 foliated, and disturbed than the soft argillites. 



In face of the facts, we seem, therefore, obliged to admit that the bound- 

 ary line between the semi-crystalline and holocrystalline portions of the 

 Maryland Piedmont area represents a great time-break. Their contact is 

 not an absolutely sharp line, nor indeed is this to be expected, since, as 

 Professor Pumpelly has recently pointed out, one formation may pass grad- 

 ually into one lying unconformably above it in consequence of superficial 

 rock decay ; * and also since any metamorphism such as both these areas 

 has undergone tends strongly to obliterate sharp lines of contact. Still, 

 while not absolutely sharp, this contact is far too abrupt to accord with any 

 supposition of a gradual or progressive metamorphism through the entire 

 series from west to east.f 



The second and third of the above-mentioned hypotheses assume the dif- 

 ference in age of the western schists and eastern gneisses and eruptives, 

 which it is the main object of this paper to establish. The second hypothesis 

 (i. e., that there was a passively resistant buttress of crystalline rock) is, 



• This volume, pp. 209-224. 



flf the boundary of the semi crystalline rocks of Maryland against the holocrystalline rocks 

 really represents a great time-break, it may se^m remarkable that basal conglomerates have not 

 been encountered near this line. These may exist, but as yet they have not been clearly identified. 

 Their presence is, of course, not necessary to prove the unconformity, although ihey are to be 

 looked for. It is not impossible, as stated on page 309, that the conglomeratic sandstone which oc- 

 curs on Deer creek, Harford county, between the gneiss and Peach Bottom slate area, may be of 

 this nature. 



