200 THE MINERAL RESOURCES OF CECIL COUNTY 



According to the recalculations of Miss Bascom given in her dis- 

 cussion of the Crystalline Rocks of the county, the average mineral- 

 ogical composition of the rock disregarding the secondary constit- 

 uents is as follows: 



Quartz 42 . 28 



Orthoclase 8 .91 



Oligoclase 34 . 86 



Biotite 10.41 



Misc 3.16 



Total 99.62 



A microscopic study 1 of sections from the Port Deposit granite 

 shows the presence of the usual granitic minerals, such as quartz, 

 feldspar, dark and light micas, apatite, zircon, titanite, allanite, epi- 

 dote, chlorite, hornblende, magnetite, garnets and occasionally calcite. 

 The quartz is in relatively large sized areas, ranging from 0.5 mm. 

 x 1.5 mm. to 3 mm. x 5 mm. With the aid of the microscope these 

 areas are seen to be not single units, but composed of a great number 

 of small quartz fragments, which have resulted from fhe crushing 

 and recrystallization of the original granite during the period when 

 the rock received its present schistose structure. These smaller 

 quartz fragments are aggregated together by intricate interlocking 

 sutures in a way which renders the rock less rigid and at the same 

 time capable of withstanding fully as much pressure as an individual 

 grain. The interstitial areas between the fragments of the coarser 

 mosaic are filled with a mosaic of still smaller grains. The feldspars, 

 like the quartz, occupy well defined areas and show the shattering 

 and recrystallization into a mosaic, as a result of the dynamic forces 

 which have modified the rock. These mosaics are much less frequent 

 in the feldspars than in the quartz. The biotite occurs in aggregates 

 of fine shreds, showing varying degrees of orientation, and is fre- 

 quently associated with irregular grains or small crystals of epidote, 

 titanite and allanite. The shreds and flakes are so small and so inter- 

 locked with minute grains of quartz, that they cause little decrease 

 in strength because of schistosity. The other constituents are so insig- 



x For a further discussion see ante pp. 117-119. 



