40 Pentti Eskola. EXIN = SE 
with groups of biotite. At places it grades over into a truly Jä 
pegmatitic variety. The crystals are tabular on (100). This 
is probably the same locality where Kotoulsky has found d 
somewhat larger crystals of orthite !), containing, according 
to an analysis of Nenadkevitsh, 3.5 4 ThOs. 
The result of a microscopical study of this orthite-bearing ; 
aplite-pegmatite showed this rock to have quite an exceptio- SS 
nal composition. The mineral constituents are: plagioclase, 
hornblende, biotite, quartz, orthite, apatite. This probably 
is the order of the quantitative proportions of the minerals 
in this rock. Potash feldspar was not at all observed. The 
"plagioclase is comparatively rich in anorthite, as 
appears already from the high refringence, all the indices of : 
refraction being higher than w in the quartz, and y, also 
higher than &. Extinction angles in sections | PM were 
found to be about 20”. Both the properties named indicate 
a mixture of about Abes,. 
The hornblen de is the most abundant constituent 
in the dark spots, forming a finely granular mass. When 
bounding the feldspar it is idiomorphic in the prismatic 
zone. It is reddish brown in colour, with the pleochroism: 
a = pale yellow < f =reddish brown <;, = reddish brown. 
The birefringence is somewhat higher than in the common 
green hornblende, as the interference colour may be as high 
as green of the second order. b = 6, c:y = 15? approx. 
The biotite shows a very dark reddish brown colour 
for « and ff. This mineral takes part in the constitution of = 
the dark spots, but it is rare as compared with the horn- 
blende. In many cases, however, it is found in the form of ' 8 
large scales scattered throughout the aplite-like mass. Apatite 
is sparingly present. ; 
It will appear from this description, that the orthite- 
bearing aplitic pegmatite has quite another composition than 
any other pegmatitic rocks of the peninsula. It is in fact 
the composition of diorite, with dominant andesine feldspar 
1 B. Kortyngbckit, MapmpyrtHea uscrxbroBaHia BE BaprysuHckoMb 
OKpPyrE BB 1912—1913 rozy, Ilerporpanae 1915, p. 41. | 
