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84 Bulletin de la Commission geologique de Finlande N: 0 212.
and even olivine-bearing norites associated with them, and all containing
hypersthene. Such rocks occurring in Central Finland were later described
by Hackman (1931) in his report on the rocks of the Nyslott section D2 of
the geologie map of Finland, and by Wilkman (1931, 1938) in the descriptions
of the rocks of the sections Kajaani C4 and Kuopio C3. Wilkman
points out that these hypersthene granites gradually pass over into granites,
as weIl as into aseries of granodiorites, diorites and norites containing both
monoclinic pyroxenes and hypersthene, and thus form transitional types in
a rock series between granites, pyroxene diorites, quartz gabbros and norites.
Mäkinen had observed similar relations with regard to the Ostrobothnian
hypersthene rocks, the granites there and the more basic diorites.
Hackman and Wilkman describe these hypersthene rocks as hypidiomorphic,
granular, rather coarse-grained rocks of a pe c u I i a r b r 0 w n
co I 0 ur: greyish-brown, greenish-brown to reddish-brown, which is
highly characteristic of this variety but unusual for granite rocks in general.
In his descriptions Hackman called these rocks »gabbro-granites», which
had been the field name used during the survey of these peculiar brownish
granites.
GEOGRAPHICAL DISTRIBUTION OF THE HYPERSTHENE ROCKS IN
CENTRAL FINLAND
Wilkman, in his descriptions of the rocks included In the map sheet
Kajaani C4 (1931) and Kuopio C3 (1938), has described a great number of
occurrences of hypersthene granites and ofpyroxene granodiorites, pyroxenequartz
diorites and pyroxene diorites associated with them. According to the
publications cited, about sixty occurrences of these brownish hypersthene
rocks are known from Central Finland. They all occur NE of a line drawn
on the map from the town of Wiborg (Viipuri) in the southeast to the town
of Piteä in the northwest, on the Swedish side of the Gulf of Bothnia. Again,
none of these rocks are to be found NE of a line drawn from the town of
Sortavala, on the north coast of Lake Ladoga, to the town of Uleäborg
(Oulu) on the northeastern coast of the Bothnian Gulf.
The hypersthene granites and related dioritic
rocks thU8 occur in a strip or belt between the two
a f 0 res a i d I i n e s run n i n g NW -SE, w h ich s tri pis abo u t
100 km b r 0 a dan d ha s ale n g t hof abo u t 450 km.
It may seem that a crack zone of about 450 km in length and about
100 km in breadth represents a rather extreme case. I should therefore
mention, as a basis for comparison, that the fault line of St. Andreas, along
which the movement of the San Francisco earthquake of 1906 took place,