MARYLAND GEOLOGICAL SURVEY 131 



Olivine is infrequently a constituent of these hypersthene-gabbros. 

 It is interesting in showing the same alteration products as the hyper- 

 sthene. A double reactionary rim surrounds the olivine wherever 

 it is in contact with the feldspar. The inner zone is colorless and 

 the outer zone composed of closely crowded very light green fibres. 

 Tremolite and hornblende, it is thought, are the two minerals repre- 

 sented. The constituents of the rim are too minute for accurate 

 optical determination, but index of refraction, double refraction, 

 color and association are those of the above minerals. 



Much magnetite and some serpentine are also developed along the 

 cracks which thoroughly penetrate the olivine. 



That serpentine, associated with the liberation and secondary de- 

 posit of silica and iron oxides, is a final product of alteration of these 

 olivinitic rocks, is believed to be the case because of the association 

 with the latter of serpentine, of limonitic opal, and of silicious limonite 

 and hematite ores. The serpentine shows the grating structure and 

 contains some tremolite, magnetite and quartz, together with an 

 abundance of hematite, limonite, opal and cryptocrystalline silica. 

 The latter group of minerals may entirely replace the serpentine. 



Typical hypersthene-gabbro showing the pyroxenes altering to 

 hornblende occurs in Grays Hill and in the meadows between G-rays 

 Hill and Iron Hill Station. 



Olivine-hypersthene-gabbro is exposed on the hill in the woods 

 south of the crossing of the road to Iron Hill Station by the Penn- 

 sylvania railroad. A typical meta-gabbro is exposed in the Penn- 

 sylvania railroad cut just southwest of this crossing and in boulders 

 along the eastern base of Grays Hill. Pyroxenite facies were not 

 found within the Maryland boundary. Serpentine and limonitic 

 opal are exposed in the railway cut mentioned, and on Mr. Jackson's 

 farm, on the left bank of Big Elk Creek, two miles north of Elkton. 



These gabbros are undoubtedly related genetically with the gabbro 

 belt to the north. They are as basic as the most basic type of that 

 belt. Throughout this southern area quartz is absent and the feld- 

 spar is bytownite-anorthite. These basic gabbros pass northeastward 

 into the acid quartz-bearing Brandywine gabbros, which cover large 

 areas in Delaware and Pennsylvania. 



