MARYLAND GEOLOGICAL SURVEY 119 



Disregarding the secondary constituents, the chief primary con- 

 stituents may be calculated as follows: 



Quartz 42.28 Orthoclase 8.91 



Orthoclase 8.91 Oligoclase, j 



Albite 23.71 Ab 2 An,, appr. \ 34- 86 



Anorthite .11.15 



Biotite 10.41 



Misc. \ Ma S nelite I 3.16 FeldSPar 43 - 77 



( Apatite . . ) 



Total 99.62 



The predominance of plagioclase over orthoclase places the granite 

 with the quartz-monzonite type. 



The term quartz-monzonite, as originally defined, covered only 

 those quartz-bearing acid plutonics exhibiting an equal development 

 of orthoclase and plagioclase. Present usage, however, gives a wider 

 signification to the monzonite. Grano-diorite has been proposed to 

 designate those plutonics intermediate between quartz-monzonite s 

 and diorites whenever plagioclase is more abundant than orthoclase. 

 It has seemed advisable to the writer to use monzonite in its wider 

 significance. 



HORNBLENDE-BIOTITE-GRANITE (QUARTZ-MONZONITE) . 



In the vicinity of the more basic igneous formation to the north, 

 the biotite-granite becomes darker colored and less gneissoid. This 

 is due to the development of hornblende, which is here the predomi- 

 nating ferromagnesian constituent. 



This facies of the granite forms a narrow zone, nowhere exceeding 

 a mile in width, between the biotite-granite and the quartz-hornblende- 

 biotite-gabbro. The type is exposed in the neighborhood of Harris- 

 ville, at the crossroads two miles west of that hamlet, and southwest 

 of Porter Bridge. 



On the southeastern border of the granite body, a hornblende- 

 granite is also developed. This is typically exposed on Principio 

 Creek, in the neighborhood of Principio Furnace, on Northeast Creek, 

 near Northeast Village, and on the lower courses of Big Elk and 

 Little Elk creeks. Feldspar, quartz, hornblende and biotite are the 

 essential constituents and can be distinguished in the hand specimen. 



