220 EDWARD B. MATHEWS 
fewer and more evenly disseminated through the quartz. The 
mineral nature of the last group could not be determined. The’ 
individual inclusions are minute apatites and zircons, hematite 
plates and magnetite. 
Quartz occurs in some of the slides as an inclusion in the 
feldspars. It is probably secondary in both the microcline and 
the oligoclase, though in the former it may possibly be original. 
With the feldspar quartz forms micropegmatitic intergrowths in 
the more weathered and crushed specimens, but this is lacking 
in the fresh, unaltered rocks. 
The feldspars in the Pikes Peak type vary in size, shape, com- 
position, and age. The color is generally pink or gray, or both 
where there is a zonal structure. The most important feldspar 
is microcline perthitically intergrown with albite. This always 
shows the characteristic ‘‘ microcline twinning” in all sections 
inclined to the brachypinacoid. The mesh of the rectangular 
grating is very small in all those instances which are regarded as 
original. In the small secondary flakes, however, the mesh is 
much coarser. 
The inclusions within the microcline are albite, quartz, oligo- 
clase, biotite, and the earlier products of crystallization. The 
most abundant are perthitic pegs of albite, and their disk-lke 
cross-sections. The former lie approximately parallel to a steep 
positive macrodome in a plane normal to the edge (001) (010). 
The small round disks may easily be confused with the pellucid 
quartz from which they can be separated only by the use of con- 
verged polarized light. i‘ 
Oligoclase is only of subordinate importance in the Pikes Peak 
type where it occurs in small light gray-green anhedral areas 
with characteristic polysynthetic twinning, lamellae showing on 
the base an extinction angle of 2°—3°. The inclusions lie close 
together near the center of the plagioclase plate and are sur- 
rounded by a zone of clear feldspar from which they are more 
or less sharply defined. The cause of the presence and position 
of these inclusions is not known. The usual explanation based 
on the increased basicity and consequent instability of the core 
