MUSEUM OF COMPARATIVE ‘ZOOLOGY. 169 
either in single crystals or simple twins, which had the optical prop- 
erties of orthoclase (although some might be triclinic) and enclosed 
between them varying amounts of a decomposed base, and of quartz 
which he regarded as secondary. The porphyritic crystals were deter- 
mined as orthoclase (sanidin). The rock varied considerably in fresh- 
ness in the several specimens. This rock, occurring thus as a surface 
flow, was called “Trachyte” under the classification used by Dr. 
Wadsworth, corresponding in this case to the “ Quartzless Porphyry ” 
of Rosenbusch. 
During the season of 1889-90 eight sections of the keratophyre and 
several sections of the detached anorthoclase crystals have been pre- 
pared for microscopic study. Numerous crystals from the groundmass 
have also been detached for the purpose of obtaining the specific gravity 
and chemical analysis. Biotite mica is often found in hand specimens, 
and occasionally augite, although the latter has not as yet been detected 
in any of the sections cut. 
In the light of our present knowledge and with further investigation 
it is possible to supplement Dr. Wadsworth’s accurate descriptions, and 
to determine the feldspar phenocrysts as anorthoclase, and the rock as a 
keratophyre. The phenocrysts occur as crystals elongated parallel to 
d, with a square cross-section owing to the presence of the base and 
brachypinacoid ; in addition to the two cleavages there is a rough trans- 
verse fissuring. The crystals are quite glassy when fresh. In the rock 
slides, in polarized light, the different feldspar sections show marked 
optical peculiarities; there is often a very fine single, or double (micro- 
cline) twinning; sometimes the whole of one section of the mineral 
consists of irregular areas not extinguishing in common, which resemble 
the phenomena produced by mechanical causes ; these areas contain 
very fine lines crossing each other at various angles in the different 
areas ; in other cases there is a very fine zonal structure. Sections pre- 
pared parallel to the base show this fine irregular double twinning, and 
give an extinction 1° to 2° oblique to the line of the second cleavage 
(o P @), and sections parallel to the latter cleavage give an extinction 
about 9° oblique to the line of the first cleavage, with an obtuse posi- 
_tive bisectrix about perpendicular to the face, the acute bisectrix a 
making the angle of 9° with the basal cleavage. These sections also 
show sometimes a very fine indistinct microperthite striation. The 
angle between the two cleavages was determined in the reflecting goni- 
ometer as approximately 89° 42’, about that of microline. The specific 
