62 



Cincinnati Society of Natural History. 



Rock Types. 



GRANITE. 



The granite which occupies most of the area studied, 

 presents considerable variety, both in structure and mineral 

 composition. As a rule, it becomes more foliated and gneis- 

 soid toward the east and south, so that the Port Deposit 

 rock has heretofore been generally regarded as a gneiss, 

 although it can be traced by slow transitions into the nearly 

 massive granite of Rowlandville. As we shall see beyond, 

 this natural difference is in great measure due to dynamic 

 agencies, which have developed secondary parallel structures 

 in certain parts of the granite mass. In the railroad cutting 

 at Rowlandville, the granite exposed is quite massive and of 

 a dark color. It is readily seen with the unaided eye to be 

 composed of a feldspar largely changed to epidote, small 

 quartz grains, and a considerable proportion of biotite, to 

 which it owes its relatively dark hue. As this granite is 

 followed northward into the valley of Octoraro Creek it 

 becomes still darker in color, through a still further decrease in 

 the amount of feldspar. This mineral now appears in rounded 

 grains; it is accompanied by about an equal amount of quartz, 

 and the rock has a somewhat parallel or gneissic structure 

 developed in it. 



Still further northward, as the gabbro contact is approached, 

 the granite develops more and more of the ferro-magnesian 

 constituents, containing hornblende as well as biotite, along 

 with an increased amount of magnetite, until, finally, very 

 near the contact, it is sometimes difficult to decide in the 

 field whether a given rock should be classified as a granite or 

 a gabbro. Numerous boulders of a sheared and squeezed 

 chloritic rock are also found near the contact. The occur- 

 rence of such apparent transitions from one rock type to the 

 other at their contact is especially noteworthy and interesting. 

 Near the gabbro contact there also occur numerous sharply- 

 defined patches in the granite. These were also observed at 

 certain other points within the granite area, and fuller refer- 

 ence will be made to them beyond. 



