Acta naturalia Islandica - 01.02.1946, Síða 57

ORIGIN OF THE BASIC TUFFS OF ICELAND
51
and the fresh appearance of the brighter sideromelan and the ab-
sence of phenocrysts, that it is not an alteration product of the darker
glass. This layer simply represents a magma that flowed out on the
surface and consolidated as glass and was very heavily charged
with an “older generation” of somewhat different glass.
The difference in colour is very clear and the limit sharp. But the
margin of the darker colour is not the same as that of the respective
area containing phenocrysts and microlites. These crystals also occur
in a sharply limited zone with the lighter colour around every dark-
er fragment. In this zone the crystals are sometimes arranged paral-
lel to the margin whereas no fluidal structure is seen in the darker
glass. In this zone even olivine is unaltered. This zoning has clearly
nothing to do with alteration but the fragments of the older glass
were laterally melted and changed in composition.
We shall not here- discuss the significance of this relation of glass
within glass which I have found at several places. But it may be
pointed out that the magma or fluid that was extruded can hardly
have been kept fluid by high temperature but more probably by a
high content of gases Avhich on rapid escape left a highly viscous
mass, unable to crystallize.
2. Brown tuff or sandstone attaining the coarseness of gravel at
the bottom. The main mass (444) consists of a chaotic mixture of
minute grains of opaque glass, brown glass, and crystal fragments
and on the whole it is closely similar to the vein (438b). On the other
hand this is very unlike a sandstone as the separate grains are in-
distinctly limited. I think it most likely that the layer was formed by
extrusion of a type represented in the above mentioned vein.
3. Conglomerate of grey clay with scattered well-worn pebbles.
This is a structureless mass of morainic appearance, although, in my
view, it is not a moraine. Scratched pebbles may be found but they
do not prove a morainic nature of the conglomerate. The grey ground-
mass (445a) consists of rounded grains of basalt, opaque glass and
basalt debris. Translucent glass may be found but it is extremely
sparse. A peculiarity of this conglomerate is, however, large and
small patches of a pink homogeneous amorphous clay of very ir-
regular boundaries, often with long protuberances into the surround-
ing mass. This layer is in my opinion a volcanic mud flow.
4. Conglomerate.
5. Grey conglomerate in every respect similar to (3).