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Cincinnati Society of Natural History. 



frequently observed. The micro-porphyritic structure is seen 

 only in a few cases, and it is probably best shown in the 

 granite which is exposed in the ravine cut by the little stream 

 one-half mile north of Frenchtown, In the thin section of 

 this granite large feldspar individuals are imbedded in a fine 

 granular quartz-feldspar ground mass, while the rock shows 

 less effects of metamorphic action than those further north. 



The intergrowths of quartz and feldspar, which form the 

 micro-pegmatite, are seen in small amount in nearly all speci- 

 mens. In the most foliated granites these quartz-feldspar 

 aggregates have been broken into small masses. The result- 

 ing fragments show the effects of pressure, especially in the 

 quartz which is optically disturbed. 



DIORITE. 



The occurrence of dark dioritic bands following in the 

 main the strike of the granite-gneiss, although in some 

 instances cutting abruptly across it, has been mentioned in 

 chapter one. There is much in the field occurrence of these 

 rocks to suggest that they are metamorphosed and uralitized 

 projections, or dykes from the large gabbro masses on the 

 north and south-west. The microscopical examination tends 

 to further strengthen this hypothesis. In their mineralogical 

 composition these rocks are now foliated diorites, or horn- 

 blende gneisses, composed essentially of secondary horn- 

 blende, plagioclase and quartz, and containing in addition 

 varying amounts of biotite, orthoclase, garnet, rutile and 

 other accessory constituents. As alteration products, they 

 also exhibit epidote, chlorite, calcite, etc. 



These dioritic bands, which are principally exposed along 

 the river bluff, are divisible into three groups. First, those 

 occurring within the town of Port Deposit; second, the three 

 belts visible between Port Deposit and Frenchtown, in the 

 vicinity of Herring Run ; third, the belt of gabbro diorite, 

 near Frenchtown, in connection with which will be considered 

 a belt of very similar rock cutting across the strike of the 

 granites near Harrisville. * 



The microscopical examination of these basic rocks shows 

 that the zone of maximum dynamic action for them agrees 

 very closely with that for the surrounding granite-gneiss. 



