oUb G. H. WILLIAMS — STRUCTUKE OF THE PIEDMONT PLATEAU. 



chemical (recrystallization) nietamorphism, though they do not lithologically 

 differ from beds which iu many other localities are known to be of Devonian, 

 Silurian, or Cambrian age. Their most important mineralogical component 

 is a silky white mica (sericite or kaolin), whose individual scales vary greatly 

 in size in different specimens. This is s-^metime? wholly or in part replaced 

 by green chlorite, which gives rise to subordinate intercalated beds of chlorite 

 slate or schist. Quartz grains of varying size and outline are generally 

 present, while feldspar is extremely rare. It is probably the alteration of 

 this mineral in the process of weathering which has given rise to the 

 abundant white mica. Iron oxide in very minute red hexagonal plates or 

 rounded grains is common, and not infrequently composes a large proportion 

 of the rock-mass. Tourmaline in minute crystals formed in sUh is very com- 

 mon even in the less crystalline varieties, while the ultra-microscopic rutile 

 needles (the Thonschiefernadeln of the Germans) are everywhere abundant. 

 In certain bands of these rocks, ottrelite is also finely-developed. 



The phyllites have always a peri'ect cleavage and a satiny luster, which 

 increases with the crystallization of new mica. Their color ranges from 

 black through every shade of purple, blue and green to a pale gray. The 

 darker varieties are largely worked as roofing slates. Evidences of clastic 

 structure are not infrequently preserved iu the shape of rounded grains 

 and small pebbles of varying appearance and composition. Where least 

 disturbed these slates are jointed and cut by cross-seams of chlorite or 

 quartz. When they are more disturbed they become greatly puckered and 

 filled with veins and eyes of quartz.* 



The most clearly defined varieties of the Maryland phyllites thus far rec- 

 ognized may be enumerated as follows : 



a. Clay slate (argillite): Along the eastern side of Frederick valley; 



b. Roofing slate: Delta and Ijamsville, where it is quarried ; 



c. Sericite slate or schist ( hydromica schist ; talcose slate of Tyson and 

 the older geologists) ; 



d. Chlorite schist ; 



e. Ottrelite schist :t between New Windsor and Liberty, Frederick county. 

 Sandstone. — The sandstone of the western or semi-crystalline area always 



retains unmistakable evidence of its clastic origin. It is most typically dis- 

 played in the southwestern corner of the area on Sugarloaf mountain, whose 

 mass it composes. It also extends in a series of isolated and unimportant 

 patches north-northeastward from the mountain (parallel to Catoctin) until 

 it finally disappears beneath the Newark red sandstone. On the east side 

 of Sugarloaf its sandstone passes through gradual transitions of argillaceous 

 sandstone and sandy slate into the overlying slates and schists. The detrital 

 character of this rock, although plainly visible to the naked eye, is much 



* It is these quartz veins, near the ea>*tern border of the semi-crystalline area, that carry the gold 

 of Montgomery county. See S. F. Emmons : Trans. Am. Inst. Min. Eng., Feb., 1890. 

 t Johns Hopkins University Circulars, no. 75, vol. VII [, 1889, p. 100. 



