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Suomen Geologinen Seura. N: 0 35. Geologiska Sällskapet i Finland. 19
Fig. 2. Diabase dike at the railway cutting W of Keuruu. The
length of the arrow is 10 cm. The country rock is porphyritic
granodiorite. Photo E. Halme.
The diabase is dark grey, often black, and its contacts with penetrated
rocks are usually quite sharp. SW of the Orion factories, however, the porphyritic
granodiorite is brecciated by the penetrating diabase.
The dikes near Keuruu are usually only a few cm in width. The widest
one (3 m in width) has been found on the eastern shore of Lake Tarhianpohja.
This dike can be followed some hundred meters, not, however, as a
dike but as a channel, owing to the complete wearing off of the diabasic
material in the course of time. This erosion took place easily because the
said diabase has excellent cleavages in several directions, mainly semiperpendicular
to the strike of the dike. At the railway cutting about 1 km E
of the church of Keuruu, a determination was made of the cleavage of the
diabase (Fig. 2). There are two parallel dikes, one 2 m, the other 90-100
cm in width. The best plane of cleavage had developed vertically to the
strike of the dike dipping 30° at N 45° W. Another poorer cleavage plane
dips 60° at S 45° E. These directions of cleavage may not be connected with
tectonic movements, but are rather to be considered directions of natural
cleavage of the rock.