Rit (Vísindafélag Íslendinga) - 01.06.1923, Blaðsíða 114

110
In the clay strata (2) are the most common species that
live on a soft clay bottom (Leda pernula, Nucula tenuis,
Yoldia limatula, Axinus flexuosus). These species still live
all around Iceland except Yoldia limatula, which is not found
on the south coast. The southern limit for this species, on
the west coast, is in Faxaflói.
At the top of the clay strata (2) and in the Iower part
of the gravel strata (1), six boreal species have been found;
Buccinum undatum, Littorina rudis, Anomia squamula, My-
tiius modiolus, Cyprina isiandica, Zirphæa crispata. The
last-named species has not been found living in present
times further north than the west cost of Iceland.
The clay strata of the terraces (2) are without doubt
formed at the greatest depth; the species of molluscs found
in them (Yoldia limatula, Nucula tenuis, Leda pernula) need
a greater depth than the other species in the terraces, in
order to develop. Around our coasts they are not common
at a depth of less than 20 m.
The clay strata in which these shells are found reach at
most 20 — 27 m. above the present sea-level. It ls therefore
very likely that these strata were deposited when the sea-
level was stationary at the 40—50 m. level, where the line
of demarcation is between the higher and lower terraces
(pp. 62-63).
The shallow water formation on top of the clay strata
(1), with the boreal species, was deposited later on, when
the sea was receding below this boundary.
If a separate advance of the sea took place, reaching its
limit at the 40-50 m. mark, it is probable that all these
shell strata were then deposited.
c) Old shell-bearing strata to the south of Hafrafjall
(pp. 84-87).
In this district the author discovered fossil shells in 12
places. Most of the shell-bearing strata were found only
1 — 10 m. above sea-Ievel, at one place only (Laxá), 20 m.
above sea-level.