k.nkois FORMATIONS. 305 



mutually affected to some degree. In any case it is certain thai the crust 

 formed upon ;i basic lava stream <>l' such thickness could effectually retard 

 the consolidation of the interior portion and give time for the formation <>(' 

 ;i highly crystalline mass 



Dccription. — The constituents of the Table Mountain basall are the same 

 as of the rocks above described, bul there is naturally ;i greal differ- 

 ence in their development in differenl parts of the sheets. Porphyritic 

 structure is more pronounced than in the <lil<<- rocks, yel the interior parts 

 of both How- are highly crystalline. The grain of the groundmass 

 produces the chief difference in outward appearance In the central 

 portions of both sheets tli<- groundmass is clearly ;i crystalline ma 

 dark-gray color. Approaching the surface the rock i- darker and the 

 groundmass becomes aphanitic. In the inner parts of the vesicular zone a 

 lens still shows a fine crystalline groundmass, and it is only in the outer, 

 more or less scoriaceous crust thai the microscope reveals a globulitic, 

 ■_■ I . • - - ■ bi 



The phenocrysts of these surface flows are nearly tli<- same in develop- 

 ment in all parts of the mass* plausible evidence thai thej existed in the 



magma ;it the time of eruption. They ;n'<- ;il~" very similar t<> those of 

 the dike rocks. Olivine is less abundant, :m<l it- crystals are now almost 

 wholly decomposed, yielding a dark-green serpentine. A.ugite has usually 

 ;i very well-developed crystal form, ;i fact which seems t<> indicate thai the 

 phenocrysts of the Ralston <lik<- became irregular through resorption 

 Plagioclase tablets like those of the <lik'- rocks are presenl here, bul less 

 abundantly. Biotite appears in the vesicles of the lower sheet in hexagonal 

 leaves attached l»\ the edges and inclosed by zeolitic deposits. 



The groundmass minerals of these sheets are plagioclase, orthoclase, 

 augite, magnetite, and apatite. Plagioclase occurs in stout little crystals or 

 staves. In the coarser-grained parts orthoclase is found in an irregular 

 oriented zone about many plagioclase crystals, as well as in irregular 

 grains. With a decrease in size of the plagioclase staves the oriented 

 growth diminishes, and in the denser parts one can only see thai a colorless, 

 apparently feldspathic substance occupies the main space between staves <>( 

 plagioclase. Irregular grains of augite and a magnetite dusl are sprinkled 



MOM XXVII 20 



