120 THE CRYSTALLINE ROCKS OE CECIL COUNTY 



Magnetite, apatite, zircon and titanite are accessory, and garnet, 

 epidote, chlorite, calcite and muscovite, secondary constituents. 



The feldspar varies from a very subordinate amount to nearly 45 

 per cent of the rock. It is both orthoclase and oligoclase (Ab 3 An^. 

 There is about double the amount of the latter. This feldspar shows 

 extended alteration into epidote, which forms crowded aggregates 

 of well-defined crystals or large irregular areas. 



An analysis of the rock from Rowlandsville gives a larger per- 

 centage of lime than the oligoclase, determined by optical measure- 

 ments, demands. This excess of lime and the widespread epidoti- 

 zation suggests the addition to the rock of lime from external sources. 



Quartz varies considerably in amount and may constitute as much 

 as 33 per cent of the rock. It is the normal granitic quartz. Partial 

 or complete granulation has affected it in many cases. 



Biotite and hornblende may be present in nearly equal amounts 

 in this type, or either one may partially replace the other. Biotite 

 is the usual deep brown pleochroic variety. 



Hornblende is a compact green varity, showing under the micro- 

 scope the deep blue green, olive green and light yellowish green axial 

 colors. In the more gneissoid granite it occurs in slender prismatic 

 crystals without terminal planes. It also occurs in broadly prismatic 

 forms exhibiting no pressure arrangement. 



CHEMICAL COMPOSITION. 

 SiOo 66.68 RECALCULATION. 



Fe 2 3 1.58 



FeO 3.23 



MgO 2.19 



CaO 4.89 



Na 2 2.65 



K 2 2.05 



H 2 + 1.09 



H 2 0— 16 



Ti0 2 50 



P,0 5 10 



MnO 10 



BaO 08 



SrO trace 



Li 2 trace 



Quartz 33.82 



Orthoclase 12.26 



Albite molecule 22.15 



Anorthite molecule 7.80 



Biotite 17.83 



Apatite 23 



Magnetite 2.30 



Titanite 1.20 



Misc 3.20 



100.79 



100.23 



1 Analysis made by W. F. Hillebrand in the laboratory of the U. S. Geol. Surv. 

 Grimsley, loc. cit., pp. 88-89. 





