MARYLAND GEOLOGICAL SURVEY 121 



The above is an analysis of a basic granite collected from this belt 

 by G. P. Grimsley, who describes it as a biotite-granite, " containing 

 an abundance of idiomorphic plagioclase crystals, with zonal arrange- 

 ment of epidote, considerable unstriated feldspar, which also contains 

 epidote and abundant quartz." 



A recalculation of the analysis was made in the light of this descrip- 

 tion. The plagioclase feldspar was given the composition of Ab 3 An x 

 which is about the composition of the feldspar of the slides from this 

 belt examined by the writer. 



This recalculation probably gives too high a percentage of quartz, 

 as some of the silica is combined with the lime, of which there is a 

 considerable percentage (2.86) remaining after the calculation of 

 the anorthite, apatite and titanite. It is believed that lime and silica 

 have been added to the rock by percolating water, thus assisting in 

 the general epidotization of the rock. 



The reduction of the potash feldspar and the predominance of the 

 lime-soda feldspar, places this " granite " also with the quartz-mon- 

 zonites. 



GrABBRO. 



META-GABBRO OR QUARTZ-HORNBLENDE-GABBRO AND HORNBLENDE- 



GABBRO. 



The belt of basic igneous material immediately to the north of the 

 quartz-monzonite, is in the main a norite and hypersthene-gabbro, 

 with a quartz-hornblende-gabbro facies along the granite contact. 



About one mile below Conowingo Station on the Susquehanna 

 river, a blue quartz is somewhat abruptly and conspicuously developed 

 in the gabbro. This is associated with the presence of hornblende. 

 The rock thus has the aspect of a quartz-diorite, and was described as 

 such by Dr. Leonard. Optical and chemical determinations show 

 that the feldspar remains, as in the norite and gabbro, a labradorite- 

 bytownite. This facies, therefore, still belongs in the gabbro family. 

 The hornblende often shows itself, by its relation to a pyroxenic core, 

 to be of a secondary character. 



The type is well exposed on the Susquehanna river, in the neigh- 

 borhood of Porter Bridge and on the left bank of Octoraro Creek, 

 near its junction with Stone Run. 



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