MARYLAND GEOLOGICAL SURVEY 123 



gation of epidote and zoisite crystals, in which the complete alteration 

 results, is the cause of the opaque white character which the feldspar 

 so generally possesses. The periphery of the feldspar crystal is often 

 left more or less free from the epidote grains. The proportion of 

 feldspar varies considerably, but rarely exceeds two-fifths of the rock. 



Hornblende always exceeds the biotite and both together nearly 

 equal the feldspar. It is the green compact variety. The prismatic 

 zone is usually well developed, giving rise to columnar forms. Con- 

 fused aggregates and imperfectly developed individuals are also usual. 

 Twinning is not uncommon. The pronounced pleochroism is of the 

 type peculiar to green hornblende. 



Magnetite, apatite and quartz may be included in the hornblende. 

 Chlorite is its alteration product and is commonly but not extensively 

 developed. 



Biotite is sometimes present in these gabbros. Its presence is 

 associated with a proximity to the biotite-bearing quartz-monzonites. 

 Like the hornblende it alters to chlorite. 



Quartz, when present, may constitute a fifth of the rock. It is 

 conspicuous in the hand specimen where it possesses a genuinely blue 

 color. It exhibits no variation from granitic quartz. 



Pressure effects are apparent both in the hand specimens and slides, 

 but only locally and in a very limited way, is anything approaching a 

 schist produced. 



Epidotization and zoisitization are the most marked metamorphic 

 changes and seem to be a result both of dynamic action and of pro- 

 cesses of weathering. 



There is a considerable increase in the acidity of the feldspar in the 

 quartz-bearing gabbros, from Ab x An 4 to Abi An x . The feldspar 

 remains a labradorite, and the rock is still of the gabbro type and not 

 a diorite. 



Orthoclase was an accessory constituent in all the slides examined 

 by the writer. It is probable, however, that some of the potassa 

 should be referred to the albite molecule, thus reducing slightly the 

 percentage of orthoclase. 



