136 THE CRYSTALLINE ROCKS OF CECIL COUNTY 



These intrusives are, for the most part, confined to the granite- 

 gneiss and are exposed on the Susquehanna between Port Deposit and 

 Frenchtown. 



Their location, extent, strike and field characteristics have been 

 discussed (pp. 97-101). Their petrographic characters will now be 

 presented. 



META-RHYOLITE. 



The most considerable and important of these later intrusives is the 

 fine-grained acid formation which has so extended an exposure in the 

 neighborhood of Bay View and on Principio Creek. The true char- 

 acter of the rock is obscured by the secondary development and 

 growth of hornblende, epidote and chlorite. These minerals give 

 a greener color, a less aphanitic aspect and a greater softness than is 

 exhibited by the fresh material. The fresh rock, on the other hand, 

 is of a greenish grey color, of very hard aphanitic character, and 

 possesses inconspicuous quartz and feldspar phenocrysts. 



Its prevailing green aspect has led geologists to class it with the 

 basic types. It was mapped as a gabbro by Professor Williams and as 

 a diorite by Dr. Grimsley. 



Examination of thin sections of the Frenchtown exposures led Dr. 

 Grimsley to classify the rock as a hornblende-quartz-diorite and a 

 biotite-hornblende-quartz-diorite. He adds that there is " nothing 

 in their mineralogical composition opposed to the hypothesis that 

 they are dynamically metamorphosed gabbros." He does not state 

 the character of the feldspar upon which the classification of the rock 

 must depend. It will be seen that the acidity of the feldspar excludes 

 this rock from either the gabbro or diorite groups. 



The essential constituents are quartz, feldspar, biotite, hornblende; 

 the accessory constituents are magnetite, apatite, titanite, garnet, 

 pyrrhotite, muscovite; the secondary constituents are hornblende, 

 epidote and chlorite. 



Slides from the fresher material show a fine-grained quartz-feldspar 

 mosaic with scattering quartz and feldspar phenocrysts, and blades 

 of biotite, or of green hornblende and garnets and magnetite. The 



