THE PSEUDO FAN STRUCTURE NOT SYMMETRIC. 313 



areas, belong to the more crystalline rocks. Further northward, however, 

 there is a considerable expanse of the semi-crystalline schists on the eastern 

 side of the axis. These do not differ in any way from those on the western 

 side, except in the direction of their dip. The same general strike and dip are 

 also unmistakably displayed within the completely crystalline rocks forming 

 the eastern area ; but here there is far less uniformity in structure than is to 

 be found among the semi-crystalline strata. Many disturbances attendant 

 upon successive eruptions, dislocations and foldings, all anterior to the move- 

 ment which gave the schists and slates their present position, have left their 

 record in a much more irregular distribution and a much more complex 

 structure ; a fact which, as we shall see beyond, is of great significance in 

 the interpretation of the sections here described. 



Contrast in Character of the Eastern and Western Areas. — Aside from 

 the fundamental difference between the holocrystalliue and semi-crystalline 

 (clastic) rocks composing the two portions of Maryland's Piedmont region, 

 which the preceding petrographical descriptions have made sufficiently clear, 

 the most marked contrast between these areas is to be found in the much 

 greater variety of rock types in the eastern division, together with their 

 much more irregular areal distribution! 



The rocks composing the semi-crystalline area show evidences of consid- 

 erable, but by no means the most intense, regional metamorphism. They 

 were once, almost without exception, ordinary sediments. The intercalated 

 limestone bands, occurring abundantly north of the Baltimore and Ohio 

 railroad main stem, have been changed to fine, almost cryptocrystalline 

 marbles, seamed by parallel argillaceous layers. The sandstones are hardly 

 altered ; not even recrystallized to an appreciable extent. The slates are 

 perfectly cleaved in a direction not greatly differing from their bedding. 

 Within them have been developed, in varying degrees, sericite, chlorite* 

 ottrelite, tourmaline, rutile, and other metaraorphic minerals ; though such 

 secondary silicates as garnet, sillimanite, staurolite, and cyanite or biotite, 

 which appear to be the product of more intense metamorphism, are found 

 only within the holocrystalliue area. The rocks of the western area are not 

 on the whole so crystalline as the fossiliferous schists of western Norway, 

 and it is not improbable that their age may hereafter be definitely settled 

 on paleontological evidence.* 



In spite of the apparent constancy of their dip, the horizontal extent of 

 the semi-crystalline rocks cannot be taken as any measure of their real 

 thickness. The cleavage has much obscured the bedding, and the succession 

 must be many times repeated by sharp folds and faults. Such dislocations 

 are noAV, however, almost entirely obscured by {!) the perfect and uniform 

 cleavage, (2) the even surface to which they have all been worn down, and 



*See foot-note on p. 307. 



