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tion of these from the P-direction indicates the sense of move-
ment along the principal fractures: it is such as to tend to
widen the open fissures. The sense is such that looking NE
or N (in N-Iceland) along a principal fracture, the left-hand
side moves away. This means that in the two-axial horizon-
tal stress field that is inferred, the vertical plane of maxi-
mum pressure lies S—N in S-Iceland and SE-NW in NE-
Iceland; the plane of least pressure is at right angle to the
other.
The observations fall into three categories. First are the
chains of open stepped fissures already referred to. Secondly
it occurs that together with or closely following the fissur-
ing, also vertical faulting takes place. One then finds that
at the site of each S-fissure is a vertical wall, while hetween
the fissures is formed a slope with a number of small frac-
tures. Thus, the originally unbroken parts between the fis-
sures have responded to the vertical movement by flexures.
Figs. 5 and 6 show such faults (Réttargjá and Strandagjá,
and Undirveggur). In the case of pure faulting or if this
antedated the horizontal movement, the S-fractures would
hardly develop, and this may be the reason why these are
not everywhere found; I have not found clear S-fractures in
the Thingvellir area.
The third category are earthquake fractures. Fig. 7 shows
1—1*4 m wide lentical fissures
formed in a postglacial lava near
the farms Borgarkot and Kílhraun
in the district Skeid in 1896 (angles
estimated). Figs. 8 and 9 show frac-
tures formed near Hekla in 1912.
At Selsundslækur, Fig. 8, the di-
rection of the zone of disturbance
is about N 30°E. Stepped fractures,
75—150 m long and offset 10—15 m
against each other, cut a postglacial
lava; they make an angle of 10—12°
Fig. 7. Earthquake frac-
ture irt district Skeid,
formed 1896.