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-porfthyry (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 
