INTEKMEDIATE CHAKACTEK OF APPALACHIAJT GRAXITES 



13 



Maryland. 



Mineral composition. — The principal minerals in tlie Maryland acid 

 plutonic rocks are qnartz, orthoclase, microcline, plagioclase near oligoclase, 

 biotite, some muscovite and hornblende, epidote, and allanite, together with 

 titanite, magnetite, apatite, zircon, and occasional!}' some others, as the chief 

 accessory minerals. 



Biotite is an important constituent in all the Maryland granites. In some 

 areas biotite is replaced in part by hornblende, in others muscovite, epidote, 

 and allanite are prominently associated with biotite. Near Garrett Park and 

 Eowlandsville hornblende is an important constituent and is about equal in 

 amount to biotite. At Guilford muscovite enters with loiotite as an impor- 

 tant mineral. 



Based on mineral composition the Maryland granites are divisible into 

 the following principal types : (1) Biotite granite, under which the majority 

 of the granites of the State may be grouped. In addition to biotite the 

 granite of some of the Port Deposit, Frenchtown, Woodstock, Dorsey Eun 

 Station, Ellicott City, and Ilchester contain considerable ej^idote and allanite, 

 and may be designated as allanite-epidote-bearing biotite granites; (2) 

 muscovite-biotite granites of which the Guilford area ib the only typical 

 representative; and (3) hornblende-biotite granite. 



Texturally, there are three rather well marked types of the Maryland 

 acid rocks: (1) Even-granular massive, (2) porph3'ritie, and (3) banded 

 or schistose granite-gneiss. 



Chemical composition. — From the number of analyses that have been 

 made of the Maryland acid rocks the following are selected to show the 

 range of composition in silica, lime, and the alkalies : 



B. Table of partial analyses of Maryland and District of Columbia 

 granites. 



Port Deposit, Cecil county, Maryland. Williams, G. H., 15tli ^Ji. Kept., U. 



S. Geol. Survey, p. 672; Bascom, F. Maryland Geol. Survey, Cecil County 



Eeport. 1902, p. 119. 

 Brookville, Montgomery county, Maryland. Williams, G. H. 15th An. 



Rept., U. S. Geol. Survey, p. 672. 



