34 
ACID TYPES 
The most acid variety occurs as small dykes cutting all the other 
phases of syenite. Under the microscope it is seen to consist of phenocrysts, 
up to one-fourth of an inch long, of mierocline and ortho clase, with some 
very irregular microperthite and a few, large, rounded grains of quartz, 
set in a fine-grained holocrystalline groundmass of feldspar and quartz. 
The feldspar of the groundmass is predominantly pure albite exhibiting an 
extinction angle of 16 degrees on faces cut perpendicular to 010. Some 
orthoclase is present and considerable quartz. The only other constitu- 
ents of the rock are some scattered, minute shreds of a blue-green mineral 
that is probably an amphibole, and a few, large, altered cubes of pyrite. 
Throughout all the large phenocrysts of this section numerous fine 
grains of plagioclase feldspar are disseminated. They are of the same 
composition as the feldspar of the groundmass and always show a higher 
index of refraction than that of the phenocrysts in which they are embedded. 
These fine grains of sodic feldspar also occur in the quartz phenocrysts 
where, however, instead of being irregularly placed, as in the feldspar, 
they are concentrically arranged, paralleling the hexagonal outlines of 
basal sections of quartz (Plate III). 
The explanation of such a structure is not readily apparent. From a 
preliminary examination it might appear that the quartz and feldspar 
had crystallized contemporaneously. On closer inspection, however, such 
an explanation, in several cases at least, seems to be impossible (Plate III). 
The aggregations of fine-grained feldspar cut across the quartz at right 
angles to the hexagon faces, and spread out in irregular areas, along lines 
approximately parallel to the crystal faces. This is repeated several 
times within a single individual of quartz until a fine, concentric arrange- 
ment is obtained, as many as a dozen layers of feldspar in some cases being 
present in a quartz crystal one-eighth of an inch in diameter. The cross- 
cutting relations are particularly apparent without crossed nicols. A few 
regular lines of bubble-like, extremely minute inclusions traverse the basal 
section of one quartz crystal. These lines of inclusions do not continue 
across the concentric layers of fine-grained feldspar, nor do they appear 
in any way to control the arrangement' of the feldspar. The planes of 
these regular lines of inclusions seem to be approximately perpendicular 
to the C axis of the quartz; that is, they lie approximately perpendicular 
to the planes of arrangement of the feldspar, which are roughly parallel 
to prism faces. Some of the large phenocrysts of quartz, however, show 
typical resorbed outlines against the groundmass of feldspar and quartz 
and the quartz has very evidently been replaced, within the crystal, by 
the feldspathic groundmass. Other cases, however, show a much more 
complicated structure. No regularity appears in the distribution of fine- 
grained albite in quartz. Large areas of the latter show optical continuity, 
but no crystal boundaries as seen in Plate III are in evidence. The whole 
intergrowth is so extremely irregular that it is impossible to decide upon 
the relative ages of the two minerals. Whether such areas are the result 
of replacement or of contemporaneous intergrowth remains open to question. 
Disregarding these doubtful cases, however, evidence has been given to 
show that quartz formed as large, hexagonal crystals and that later it 
became unstable in the still fluid, highly feldspathic residuum and was 
replaced by it. 
