VOLCANICS OR, LEE MIGHIGAMME DIS TARGET. S17 
requires the production of secondary quartz and the influencing 
of it by the phenocrysts—similar to the frequently described 
oriented enlargements of quartz grains in sandstones and quartz- 
ites—it seems more natural to suppose that after the crystalliza- 
tion of the phenocrysts when the lava was extruded, there began 
a rapid crystallization of the mineral elements from the remain- 
ing magma, resulting in the production of the feldspar in very 
imperfect crystal individuals, with the quartz as a cement, the 
orientation of the latter being determined by the quartz pheno- 
crysts. In other words, the quartz continued to grow, retaining 
its previous orientation, its continuity interrupted, however, by 
the feldspar grains. 
Aporhyolites— Intimately associated with the above described 
quartz-porphyries we find rocks very similar in every way to them 
macro- and microscopically, as far as the mineral constituents 
are concerned, so that the description of the quartz-porphyries 
will answer for the aporhyolites. It is only upon the presence 
of a well-developed perlitic parting, which is taken as indicating 
the presence of an original glass, that they are classed with the 
rhyolites. These perlitic cracks are now well brought out in 
ordinary light by the chlorite flakes found along them. Between 
crossed nicols these disappear and the groundmass resolves 
itself into a fine-grained mosaic of quartz and feldspar. 
Tuffs—The only acid tuff found is formed from the rhyolite. 
On its weathered surface it is white and exceedingly rough. 
This roughness is due to great extent to the weathering and sub- 
sequent leaching out of the feldspar, giving the rock an almost 
scoriaceous appearance. Its brecciated character is admirably 
shown by the difference in weathering of the fragments and 
cement. The latter being very siliceous is more resistant than 
the rhyolite, and stands out from the rock as white ridges mark- 
ing the outlines of the fragments. Under the microscope in 
ordinary light the tuff is seen to be composed of angular frag- 
ments of rhyolite, in which is a well developed perlitic parting, 
and of angular pieces of quartz and feldspar, both orthoclase and 
plagioclase, held together by a cement of quartz. 
Conclusions. —It will be seen from the foregoing preliminary 
