Granites of Cecil County, in North-eastern Mary/and. 99 



Port Deposit Series. — In the belt of black rock along the 

 road, which leads out of the center of the town of Port 

 Deposit, a number of specimens were collected. Macro- 

 scopically, these appeared to be typical hornblende gneisses 

 (1, 8, 10). In one specimen (10) the banded appearance was 

 found to be due to secondary epidote arranged along certain 

 layers. Microscopically these rocks possess a decided foliated 

 structure, and the quartz, orthoclase and plagioclase are 

 mostly crushed into a fine mosaic. 



The hornblende shows no pressure phenomena, but it is 

 often pitted with small quartz grains, and in other ways 

 manifests its secondary origin. It is of a bright green color, 

 strongly pleochroic, a light yellow, b greenish yellow, c 

 bluish green. The absorption is c>6>a. Magnetite is also 

 present in considerable amount. Epidote is sparingly devel- 

 oped in the feldspar of sections one and eight, and so abund- 

 ant in section ten as to rival the hornblende in prominence. 

 Rutile is present in very small amount. 



Along the Rock Run road, at the northern edge of the town, 

 the dark rock is essentially like the preceding, but shows 

 somewhat less pronounced effects of crushing. The feldspars 

 exhibit only disturbed extinction and a slight peripheral 

 granulation, but they are more or less changed to secondary 

 epidote. 



Herring Ran Series. — The first of the dykes in this region 

 below Port Deposit is only a few feet in width. A thin 

 section shows characters quite like the rocks of the last 

 region, but with less hornblende, and with a considerable 

 amount of irregularly-arranged epidote. The original con- 

 stituents are completely crushed to a fine • mosaic. The 

 largest of these dykes is five hundred feet in width and shows 

 great variations in grain. The diorite toward the edge of 

 this wide dyke is a fine-grained rock, with less hornblende 

 than the other two belts. The microscope shows beautiful 

 crushing effects, which are seen in the elongated and broken 

 quartz areas. The hornblende occurs in greatly elongated 

 crystalloids, which follow closely the direction of foliation. 

 Feldspar eyes are broken into a coarse mosaic filled with 

 secondary epidote. Minute grains of titanic iron are sur- 

 rounded by leucoxene rims, and there is a considerable devel- 

 opment of secondary sphene. 



