MARYLAND GEOLOGICAL SURVEY 125 



NORITE AND HYPERSTHENE-GABBRO. 



The prevalent type of the gabbroitic belt is a norite. This type is 

 associated with a hypersthene-gabbro. Neither type is confined to 

 any particular portion of the region, and the latter may be considered 

 a mineralogical facies of the norite. 



Possessing the same constituents, structures and alteration pro- 

 ducts, they differ petrographically only in the presence in the latter 

 type of a monoclinic pyroxene in about equal proportions with the 

 orthorhombic pyroxene. They will not be separated in the petro- 

 graphic discussion. 



Good outcrops of the norite and of the hypersthene-gabbro are 

 found on the Susquehanna river, while boulders of both types strew 

 the country throughout the gabbroitic belt. The rock is not usually 

 as coarse-grained as the hornblende-gabbro, nor does it exhibit the 

 mottled character of the hornblende-gabbro, but is of a more uni- 

 formly dark tone, often possessing a bronzy or purplish tinge. The 

 shade varies from a light tint where the rock is rich in feldspar to a 

 dark tint where the rock is so poor in feldspar as to be almost a pyrox- 

 enite. Both types are remarkably fresh. 



A reddish-brown hypersthene and glossy grey feldspar are essential 

 constituents easily distinguished in the hand specimen. A monoclinic 

 pyroxene, diallage or augite, is an accessory constituent in the norite, 

 and partially replaces the hypersthene in the hypersthene-gabbro. 

 A porphyritic norite, with phenocrysts of diallage, 13 mm. in length, 

 imbedded in a hypersthene-plagioclase groundmass, has been reported 

 from the vicinity of Mt. Hope Church, one mile southwest of Sylmar. 



The accessory constituents are magnetite, apatite, quartz and rarely 

 olivine. Magnetite is less abundant than is usually the case in 

 gabbroitic rocks. The secondary constituents are hornblende, epi- 

 dote, zoisite, albite, and chlorite. 



The feldspathic constituent is again bytownite. Practically the 

 same extinction angles were found on the basal and clinopinacoidal 

 planes as in the case of the bytownite of the hornblende-gabbros. 

 Dr. Leonard reports the specific gravity as 2.74, showing that the 

 feldspar may range somewhat lower in basicity. The proportion of 



