maryland geological survey 117 



Granite-Gneiss, 

 biotite-granite (quartz-monzonite) . 



The most acid type of the igneous intrusives occurring in large 

 bodies is represented by the biotite-granite-gneiss of the Port Deposit 

 and Frenchtown quarries. This type has received considerable atten- 

 tion because of its economic importance. 



It is described and figured in Volume II of the Maryland Geological 

 Survey (pp. 138-147), where Plate VIII reproduces the surface of a 

 polished block, and Plate IX, Figure 1, shows the microstructure of 

 the rock. 



The rock is of a light bluish white to grey color, mottled with irreg- 

 ular, more or less disconnected and overlapping bands of dark biotite. 

 It is this parallel grouping of the mica flakes that produces the gneis- 

 soid structure which is so characteristic of the Port Deposit granite- 

 gneiss. 



The original constituents of the rock are quartz, feldspar and bio- 

 tite, with accessory constituents apatite, zircon, titanite, allanite, 

 garnet, tourmaline and magnetite. Secondary constituents are mus- 

 covite, hornblende, epidote, chlorite and occasionally calcite. Quartz 

 and feldspar give the predominating light tone to the rock. There 

 are considerable areas of the former mineral, which resolve them- 

 selves under the microscope into aggregates of interlocking quartz 

 grains. The granulated character and the freshness of the quartz 

 offer a sharp contrast to the greatly altered but less granulated 

 feldspar. The chemical simplicity and stability of the quartz pre- 

 vents its adjustment to pressure by means of a new arrangement of 

 the chemical molecules. The strain is relieved only by granulation 

 and recrystallization. The feldspars, on the other hand, adjust them- 

 selves to the zone of pressure not alone by granulation and twinning, 

 but also by a new chemical combination. The chemical change 

 involved in the production of the new mineral is slight, but the 

 physical change is great. The mineral formed is muscovite. The 

 production of this mineral, with its longest axis at right angles to 

 the pressure, enables the rock to occupy much less space in the 

 direction of pressure. 



