Acta naturalia Islandica - 01.02.1946, Síða 79

ORIGIN OF THE BASIC TUFFS OF ICELAND
73
the hypothesis of a subaquaeous origin of the pillow lavas and brec-
cias and other structures found here and concludes that these struc-
tures were formed without the chilling influence of water, which
agrees with the results arrived at in my paper.
At last a remark on the difference between tachylyte and sidero-
melan may be made. It is clear that it is not caused by a different
rapidity of cooling. I wonder whether it is not a sufficient explanation
that tachylyte seems always to have been formed under the direct
influence of free oxygen, either from the air or from groundwater,
with a resulting rapid formation of magnetite, whereas sideromelan
is formed without contact with free oxygen.