276 Tlic Aincrican Geologist. May, i899 
feet only on a single summit (west of Rurtherford's island on 
Linekin's neck) . 
Formation of Island. — Glacial deposits are represented by 
stray boulders only. The soil is therefore thin and bare rock 
may be seen everywhere. Rutherford's island is composed, 
like the mainland within the area of the map, (see plate VIII) 
of a folded mica gneiss. In lithological character, this for- 
mation varies from a quartz biotite schist, with little or no feld- 
spar, of medium grain and dark color, to a light colored 
coarse-grained gneiss. (Plate IX.) 
Acid Dikes. — The gneiss is crowded with pegmatyte veins 
and intrusions. These veins usually lie parallel to the strike 
and to the cleavage planes of the gneiss. They are extremely 
irregular in width, expanding abruptly into rounded knobs 
around which the envelloping rock bends — or disappearing in 
attenuated bands of almost pure quartz. Pegmatitic lenses par- 
allel to cleavage and strike repeatedly occvir. \'eins of pure 
c]uartz also abound. There are also present pegmatytes of a 
more dike-like character. These are more often parallel to the 
joint planes and transverse to cleavage and strike. The south- 
ern end of Hay island shows such an acid dike, as do also some 
of the Thread-of-life ledges. These dike-like bodies are alike 
in their extremely acid characters and in their coarse texture. 
Cutting the pegmatytes indiscriminately are basic dikes 
whose intrusive character is unmistakable. These dikes vary 
in width from two hundred feet to a few inches, but in no 
case is the grain of their crystallization comparable in coarse- 
ness to that of the acid rocks, nor is the nature of their occur- 
rence similar. 
The pegmatytes demand a somewhat different explanation 
for their formation. This may be found in the predication of 
a water-saturated magma.* The great size of some of the 
intrusives, the absence of banding or comb-structure peculiar 
to veins of purely aqueous deposit, and the order of crystalli- 
zation indicate an igneous origin. While the grain of the 
crystallization and the passage into purely siliceous veins de- 
mands the presence of water. The parent mass may under - 
*W. O. Crosby and M. L. Fuller: The Origin of Pegmatite. Tech- 
nology Ouarterlv.'vol. IX. No. 4, Dec, 1896, pp. 326-356. 
