Jökull - 01.12.1981, Page 4

DISTRIBUTION AND SAMPLING
Phe samples are all taken from lava flovvs
which include tholeiites, olivine tholeiites and
olivine basalts and dolerites. Formation of the
discontinuities appears to be commonest in
thc finer grained rocks with larger amounts of
interstitial glass.
The Irish samples are taken from rocks
which are from Eocene to Oligocene in age
whereas the Icelandic rocks sampled vary
from 12 m. y. to 1 m. y. old. In addition a
freeface of the same thickness can represent a
rather different time interval in the two
countries. There are many interbasaltic
weathering horizons within the Irish succession,
whereas several hundred metres in the Icelan-
dic succession may have erupted within a
period of only 1 m. y. 'Fhe Icelandic rocks have
also been less deeply buried.
A preliininary examination of borehole
cores suggests that probably similar discon-
tinuities occur in Icelandic eruptive rocks to
70 m depth at least.
Orientated samples were taken by extract-
ing with a minimum of force the rock prisms
fornted by the joint sets typical of the freeface
environment. Surfaces were cut using
kerosene lubricant, both along and across the
prisms which vary in size from 200mm x
250mm to 350mm x 550mm. In most cases
recognition of the discontinuities can be made
under the low power stereo microscope but
sometimes they can only be distinguished in
thin section. In the majority of cases the dis-
continuities are bonded by secondary miner-
als and the prisms often have considerable
cohesive strength.
DESCRIPTION
Morphowey and fabric
Under the stereo microscope several clearly
repetitive features can be recognised on the
cut surfaces. The colour of the lines is usually
black, or red brown where they approach the
prism edges. Samples freshly broken along the
discontinuities show correspondingly either a
Figure 1. Typical discontinuities viewed on
cut surfaces. Not to scale. (a) Joint sets, mainly
vertical. (b) Stepped discontinuity (c) Surface
defect acts as stress concentrator. (d) „Weath-
ering rind“. (e) Fracture developed behind
(d).
Mynd 1. Dcemigert srnásprungukerfi séb á skornu
sýni. (a) Aðalsprungusett, aðallega lóðrétt. (b)
Smásprunga með prepum (c) Yfirborðsgatli verkar
sem spennuauki. (d) ,,Veðrunarskorpa“. (e) Smá-
sprunga á bak vtð (d).
distinctive mat black (polarization of reflected
light?) coating, or a red brown coating with a
flaky structure.
The following features are typical of all the
rocks when cut in either plane, and are sum-
marised in Fig. 1. The discontinuities usually
completely cross the prism although in per-
haps 20% of cases they terminate within it.
There is sometimes evidence of parallelism
and then often with one of the prism sides and
since these represent joint surfaces they are
most often statistically vertical in an undis-
turbed flow. Approximately 50% of discon-
tinuities intersect. The most common angles of
intersection are at 90°, at 120° in a triple
point, or at 30°. In some cases there is no
intersection but abrupt changes in direction at
120°, which give a stepped appearance. Short
offshoots also occur at the same angle.
Occasionally a discontinuity bends to inter-
sect another at 90°. There is never any dis-
2 JÖKULL 31.ÁR