GENERAL GEOLOGY 
35 
Si 0 2 . 57.62 
Fe„O s , Al a O s , MrijO,. 24.47 
CaO. 8.74 
MgO. 1.34 
K 2 0 .30 
Na 2 0 . 1.00 
Loss on ignition (H a O). 8.58 
102.05 1 
The basalt of the dikes is a very fine-grained dull black rock, con¬ 
taining veins of chalcedony and calcite. It has an exceedingly fine 
ophitic structure, with much magnetite. 
The other specimens were found as beach pebbles. 
Afilite ( 164 ). A yellowish-white compact rock, much finer grained 
than the aplite from Plover Bay (described later) but much like one 
from St. Lawrence Island. It shows, over the whole slide, an ex¬ 
ceedingly beautiful fine-grained micropegmatitic groundmass surround¬ 
ing small feldspars with raveled-out ends. There are occasional larger 
phenocrysts of oligoclase, some quartz grains, and shreds of biotite. 
Pyroxene -tonalite ( 161 ). A fresh and wholly granitoid rock of 
rather coarse, even grain, and with a pearl grey color dependent on the 
fact that the feldspars which make up most of the rock are blackish 
translucent in the central half and opaque flesh-colored on the outside. 
It contains much pyrite and little of any dark constituent. The feld¬ 
spars are largely idiomorphic and strongly zoned. They are near albite- 
oligoclase, and are completely kaolinized inward to a sharp boundary. 
They are surrounded by a very complete micropegmatitic structure, 
which radiates out from them, growing coarser outward, and the lobes 
of the feldspar are seen as well in common light as with polarized 
because of the kaolinizing. The dark constituent is a pale pyroxene, 
nearly all changed to hornblende. 
Augite-forphyrite ( 163 ). There is a brownish-red base with small 
brick-red porphyritic feldspars. The microscope shows it to be quite 
fresh, but with fissures and cleavage planes filled with red rust. The 
feldspars occur in very broad twinning bands, with extinction 38°-39° 
on the albite twinning plane, and so are very basic labradorite. The 
ground is hyalopilitic with a remnant of devitrified glass. 
1 Fe was weighed as Fe 2 0 3 when some of it possibly was FeO. Mn was 
weighed as Mn 3 0 4 when it undoubtedly existed as MnO* This brings the total 
above 100 per cent. 
