Greinar (Vísindafélag Íslendinga) - 01.01.1977, Síða 122

120
Subsidence is thus an unavoidable assumption for the bord-
ers of the South Atlantic. With this and our analysis of North
America in mind, we can state this: In geography and geo-
logy there is an old division, which has grown to a dogma,
namely that there are two main fundamentally different types
of coasts, the Atlantic type and the Pacific type. As pure
description, this is quite right but the difference of the types
is in no way fundamental. It is simply due to the change of
drainage in the Cretaceous as far as North America goes. And
we have good reasons to expect the same or similar exogenous
causes for other Atlantic coasts.
One might try to trace Mesozoic and older coastal structures
along coasts of the Atlantic type, as has been done along the
east coast of North America: But in this connection we must
keep in mind the results on the formation of deeper seismic
layers in Chapter 5, cf. also the failure to trace Hercynian
trends from Britain to New Foundland, although that might
be due in part to a cover of Lower Tertiary basalts, as stated
before. We remind also again of the total ignoring of geologic
structure in the seismic layers through the Sierra Nevada
(Chapter 5). The Tertiary formation of seismic Layer 3 in
Iceland, in the Oligocene, as well as horizontal seismic layers
right through the roots of the Alps, also cast very much
doubt on the possibility of detecting geologic structure be-
low the sea floor. On the other hand, Mesozoic sediments along
the east coast of North America have behaved differently, so
one need not be too pessimistic.
continents, have been taken to correspond to real and physicaily mean-
ingful movements (83).
To illustrate the capability of kinematics, we point out what any reader
can verify, that the Andesite line in the Pacific, with its Tonga comer and
a continuation south along New Zealand, and continued by a seismic line
towards the East Pacific Rise, is very similar to the east coast of South
America. A computer can easily perform the kinematic feat of lifting
South America and find a very good “best fit” between this Andesite line
and the east coast of South America. But who would think that this kine-
matic feat corresponds to a real event in geological history?