VOLCANICS OF THE MICHIGAMME DISTRICT. S15. 
groundmass is then seen to be made up of reticulating areas of 
clear quartz, in which lie imbedded irregular pieces of feldspar. 
The quartz occurs in irregular areas, extending in all directions 
in long slender or short broad portions, these extensions jointing 
again farther on or being connected by crosspieces, all enclosing 
irregular pieces of feldspar. The network of quartz is best 
brought out when it shows its highest polarization color, as then 
the feldspar is for the most part dark. The pieces of feldspar in 
such a quartz area do not all have the same orientation, as is 
shown by the varying extinction angle, though a goodly number 
do extinguish simultaneously. The boundaries between the 
quartz areas, which can only be seen between crossed nicols, are 
bridged over in some instances by the pieces of feldspar. The 
main point is that the quartz is the base, the feldspar imbedded 
in it, and that in those uniformly polarizing quartz areas the 
included feldspar pieces show varying optical orientation. 
Therefore, the structure is not a true micropegmatite, according 
to the generally accepted definition. 
Around the porphyritic quartzes there is a zone having 
exactly the same structure as the groundmass, and the connec- 
tion of the quartz of the crystal and that of the zone is well 
shown by the continuation of the quartz with it, and the con- 
sequent agreement in orientation (PI. Il., Fig. 2). The lack 
of a uniform optical orientation of the feldspar fragments is 
made especially apparent when the quartz is cut perpendicular 
to the c axis, and consequently remains dark between crossed 
nicols. Under the above circumstances we see certain feldspar 
grains polarizing in the zone around the quartz, and as the stage 
revolves new fragments lighten as those which polarized in the 
previous position of the stage become dark. The feldspar seems 
to have no effect upon the groundmass. It certainly is never 
surrounded by zones, as is the quartz. An explanation for these 
micropoikilitic zones should, then, also explain the structure of 
the groundmass. 
The irregular areas in the groundmass are certainly in some 
cases, and probably in most cases, the result of tangential sec- 
tions through one of these micropoikilitic zones surrounding the 
