GENERAL GEOLOGY 37 



to the right, rests on a dark cindery tuff below and is 

 itself covered to the right by a thick bed of dark, ap- 

 parently basaltic, lava. On nearer inspection, the great 

 dike and the bed associated with it proved to be so full of 

 inclusions as to resemble a coarse tuff, but the relations 

 indicated by the dotted lines in the figure seem to be the 

 true ones, and the great columnar mass seems to have been 

 erupted through the tuff bed below, taking up many frag- 

 ments from it and pouring out over the surface to form 

 the bed to the right. 



The lower bed is made up of large blocks of various 

 dark rocks, basalts and andesites, with cavities filled with 

 large masses of chalcedony, jasper, and amethyst. 



One is a typical jet-black basalt of ideal freshness, the 

 larger generation of plagioclase just visible. Another is 

 a dark aphanitic rock with distinct square phenocrysts of 

 plagioclase and few augites and olivines in a hyalopilitic 

 groundmass. It may lie between basalt and andesite. 



Other brown, brick, and red blocks are altered basalts 

 so full of calcite and chalcedony that their original struct- 

 ure is disguised. 



The augite-andesite (153) is porphyritic with abundant 

 plagioclase (anorthite), rare augite, uralite and magnetite. 

 The groundmass is fine hyalopilitic. 



The hornblende-andesite-porphyry (158) has a dark 

 green ground and is rather coarsely porphyritic. The 

 groundmass is quite coarsely holocrystalline. The horn- 

 blende is in part basaltic and resorbed, in part uralitic as 

 if from augite. Its cavities contain great geodes of fine 

 amethysts and many thick veins of jasper and agate. 



The rock which constitutes the upper bed (152) is light 

 grey, having the aspect of a trachyte, but so full of minute 

 fragments of various rocks that it is difficult to determine 

 the original porphyritic constituents. The mass of the 

 rock is a colorless glass full of fine brown dust, often 



