BASIC VOLOANIOS OP HEMLOCK FORMATION. 



103 



nature of the groundmass is afforded by the groundmass, which, under the 

 microscope, shows variable lighter and darker shades of bi-own, and these 

 portions interpenetrate, forming an imperfect eutaxitic structure. Such 

 structures are especially common in the glasses. The photomicrographs 

 (figs. A, B, PI. XXVI) show very well the general microscopical characters 

 of this rock. 



Chemical composition. — Tlie followlug complete analysis, for which I am 

 indebted to Dr. Henry Stokes, of the United States Geological Survey, 

 shows the chemical composition of one of these pre-Cambrian nonporphy- 

 ritic metabasalts. The rock is very fine grained and microophitic, with a 

 marked amygdaloidal character. The amygdaloidal cavities are filled with 

 chlorite, into which project crystals of epidote-zoisite and calcite. The 

 altei-ed condition of the basalt is very clearly shown by the high percent- 

 ages of water and carbon dioxide. The other oxides show nothing 

 remarkable except that the percentage of titanium oxide is quite high. On 

 the whole, however, the analysis is very similar to those of recent fresh 

 rocks of the same character. 



Analysis of pre-Cambrian nonpar phyritic metabasalt. 



Constituent. 



SiOj 



TiO.2 



AI.2O3 



FejO; 



PeO 



MnO 



MgO 



CaO. 



Na,0 



46.47 

 1.28 



16.28 

 3.15 

 8.96 

 .09 

 6.56 

 7.90 

 3.64 



K2O 



P,Of, 



CO2 



CI 



SO3 



HoOatllQo 



HcO above 110°. 



Total 



0.21 

 .13 

 1.26 



3.89 

 100. 11 



PORPHYKITIC METABASALT. 



The porphyritic rocks are fine grained, and may be or may not be 

 amygdaloidal. They include diabase porphyrites and porphyritic forms of 

 the melaphyres. These last in the textures of their groundmass correspond 

 to the labradorite-porphyrites, the equivalents of the andesite-porphyries, 

 though more basic than they. The phenocrysts lie in a fine groundmass 

 which shows the same kinds of texture already mentioned as having been 



