MARYLAND GEOLOGICAL SURVEY 137 



hornblende rapidly increases in amount upon the periphery of the 

 dike and in all altered material. 



The quartz-feldspar mosaic seems to be an original feature and not 

 due to the complete crushing of the constituent of an originally 

 coai se-grained rock, as premised by Dr. Grimsley. 



A proof of this lies in the presence of uncrushed phenocrysts, some- 

 times with sharp crystal boundaries, which possess inclusions of the 

 fine mosaic of the groundmass. 



Quartz constitutes a considerable proportion of the rock; not far 

 from forty per cent when the rock is fresh. 



Feldspar is somewhat less in amount. It is rarely twinned and 

 is without crystal boundaries. As a constituent of the groundmass, 

 it is too fine-grained for determination. So far as the small, imper- 

 fect and scattering phenocrysts admitted of determination, orthoclase, 

 albite and an acid oligoclase are the species represented. Biotite 

 occurs in numerous brown plates. It decreases in amount with the 

 appearance of hornblende and disappears altogether with an excess 

 of that constituent. Hornblende appears as sheaves of acicular green 

 or nearly colorless crystals, or as minute acicular crystals, distributed 

 with uniformity and parallelism, and constituting the prevailing con- 

 stituent of the rock, or as broad green blades, also showing a parallel 

 arrangement. 



Along the periphery of the dike, as exposed on the Susquehanna, 

 an excess of hornblende is developed in the rock. It is of an acicular 

 character, passing, toward the center of the dike, into the sheaf-like 

 aggregates. 



Where widespread alteration has occurred, as is the case in all the 

 exposures northeast of the gorge of the Susquehanna, notably at 

 Blythedale and Bay View, hornblende is the prevailing constituent, 

 or hornblende and epidote are the prevailing constituents, and horn- 

 blende in either case occurs in broad blades. 



The Bay View greenstones are composed of hornblende and epi- 

 dote, with a little chlorite, which constitute the mass of the rock. 

 Scanty interstitial quartz and feldspar still remains. 



It can hardly be questioned that lime has been brought to the 

 rock and thus assisted in the formation of these secondary constituents. 



