Tertiary Volcanic Plateau. | 389. 
fragmental product in, fissure eruptions, but this objection is removed 
after a study of the OdaSahraun deposits. It must be remembered 
that there is commonly some explosive action accompanying the 
extrusion of the lava, although the fragmental material thus formed 
is much inferior in quantity to the latter., Basic tuff cones 
500 metres high have been built up in the Oda%¥ahraun, and by 
attrition of these and smaller masses the supply of loose material 
blown about the desert is constantly reinforced. It is not necessary 
to imagine the red parting to be derived from fragmental material 
erupted during the extrusion of the underlying basalt; it may belong 
to an outburst far removed in place and time, or several such. 
We conceive of these deposits being transported hither and thither 
over the plains till they happen to be caught and imprisoned by an 
outpouring of lava. The rising of a wind a few hours before eruption 
may make all the difference as to whether a basalt is to be accompanied 
by a tuff or not, and by this interpretation of the red partings we are 
guarded from attaching too much importance to their occurrence and 
thickness, which may be purely accidental.’ 
The flooding of basalts and volcanic sand in the manner postulated 
has been graphically described by W. L. Watts (6, pp. 148-56), who 
had a unique view of the last fissure eruption which took place in the 
‘ Myvatnsoreefi’ (a northern continuation of the Oda%ahraun) in 
August, 1875. Sometimes thin bands of acid tuff are intercalated 
in the red partings. They may be mere streaks an inch thick, 
standing out by reason of their lighter colour, and remarkably 
persistent for considerable distances. One band two inches thick in 
a parting at Seydisfjord contains well-developed crystals of soda- 
microcline and soda-pyroxene, minerals common in Icelandic acid 
rocks. In such cases where the bands are purely acid and contain no 
basic material they obviously result from a contemporaneous eruption, 
and it is also clear that the palagonite rock above them is material 
_ blown from a distance. 
There is one feature that distinguishes the dust deposits of the 
modern Icelandic deserts from their Tertiary equivalents, and that is 
their colour. The dust is usually brown, but I have never observed 
the bright-red tint so characteristic of the old red partings. The 
colour of these Tertiary rocks is generally taken as an indication that 
they were formed under warmer climatic conditions than obtain in 
Iceland at the present day, but that this is not acomplete explanation 
will be evident from the following extract of a letter I received from 
Professor Thoroddsen in January, 1914: ‘‘You will see the red beds 
almost everywhere in the basalt formation, but not only there; they 
are also common between the younger dolerite flows which are 
striated, and probably were outpoured during the Ice Age. I have 
not done any special investigation of these red beds, but I think they 
most frequently must be deposits of volcanic ash, and seldom resulting 
1 “*Tf the occurrence and thickness of this layer’? (i.e. the interbasaltic red 
parting) ‘‘ could be assumed as an indication of the relative lapse of time 
between the different flows of lava, it would furnish us with a rude kind of 
chronometer for estimating the proportionate duration of the intervals between 
the gpupuene. (1, pp. 254-5). 
