MICROSCOPIC STRUCTURE OF PIEDMONT QUARTZITES. 321 



fication, but eastward the schists are very much puckered. The area is 

 broken through in several places by Mesozoic diabases. 



East of the axis the rocks are contorted gneisses with general westerly 

 dips, nearly perpendicular at first, but by degrees assuming a lower and lower 

 angle. Several narrow serpentine belts traverse these rocks, which are 

 also disturbed in numberless places by later eruptive granites. The latter 



FiGUKE 4.— Thin Section of Sugarloaf Sandstone X 25. 



Showing the rounded grains of quartz which exhibit undulatory extinction, and the unchanged 

 interstitial materials. Between crossed Nicols. 



Figure 5— Thin Section of Quartzite from Baltimore County X 25. 



Showing contiplete recrystallization of the siiieious matter, and also mica plates. The undu- 

 latory extinction in the quartz is very marked in this case also. Between crossed Nicols. 



exhibit typical hornblende varieties at Garrett Park and elsewhere. From 

 all appearances the gneiss area was originally largely granitic, but through 

 the agency of enormous orographic pressure has been squeezed into its pres- 

 ent gneissic condition. This is well shown by the microscopical examination 

 of thin slices of the rock. The mineralogical constituents all present great 



