COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

54 Bulletin de 1a Co=ission geo1ogique de Finlande N: 0 212.
Aeeording to Frosterus (op. eit. pp. 44-45), the pegmatite-like younger
granites of Asikkala, Heinola and Mäntyharju are likewise orthoclasebearing.
In another paper (Frosterus 1902 b, pp. 4-7), he refers to so me
medium- and eoarse-grained granites as weil as certain porphyritie varieties
as containing orthoclase besides eonsiderable mierocline; similarly, monoeHnie
potash feldspar should oeeur in addition to cross-hatched mieroeline
in some augen gneisses belonging to the basement of the Ladogian sehists
in Carelia (p. 20).
Foilowing the literature in ehronologieal order, the next mention of
orthoclase is made by Borgström (1907), who found monoclinie potash
feldspar as insets in the granite porphyry dikes at Östersundom.
In the explanatory notes for the map sheet (1 : 400 000) of Tampere,
Sederholm (1913) repeats the orthoclase observations whieh he had made
already in 1899. In addition, he mentions the following localities: 1. Orthoelase
porphyries at the lakes of Vaavu- and Varvusjärvi (Teisko), at Valkeajärvi
(Orivesi) and at Pyljyhuhdanniemi (Messukylä) (op. eit. p. 36); 2. Post­
Bothnian porphyritie granites on the boundary between the communes of
Süpyy and Merikarvia (op. eit. pp. 74, 77).
In 1917 there appeared, however, a very remarkable work on the potash
feldspars by Mäkinen (1917), a paper which, in its time, was probably the
most important contribution to the study of feldspars. According to Mäkinen,
the occurrence of orthoclase in Finnish granites, other than rapakivi, is
most questionable. Furthermore, Mäkinen claims for all the cross-hatched
microclines either an unhatched triclinic or a monoclinic ancestry, thus
being secondarily formed possibly from orthoclase. T heu n t w i n n e d
ins e t s 0 f pot a s h fe I d s par occurring in the porphyritic granites
of Central Finland have, however, a c c 0 r d i n g toM ä kin e n, g r 0 w n
p r i m a r i I y a s m i c r 0 c I i n e.
The presence of orthoclase in the crystalline schists of Finland was
definitely denied by Mäkinen.
The localities mentioned by Väyrynen ((1920, 1923) are of particular
interest because they are close to those localities in Central Finland where
the special study by the authors of the present paper has been predominantly
carried out. Therefore we will take here, as an example, one of the detailed
descriptions of Väyrynen (1920, p. 27): In the village of Lahdenkylä (Parish
of Dmajoki), there is a deeply disintegrated alkali ne granite, which, when
fresh, is grayish green in colour. The granite is coarse-grained and porphyritic,
containing lens-shaped insets of orthoclase, up to 6 cm across, in a matrix
with black amphibole, albite and quartz. Orthoclase has a homogeneous
extinction which is straight on the basal plane, but has an angle of extincti
on of 5.0 0 on the clinopinacoids. In addition, the rock contains small grains
of highly cross-twinned microcline.