The Botany of Iceland - 01.12.1914, Qupperneq 28

212
THORODDSEN
and Antrim upon chalk; in Iceland, where the basalt formation has
a thickness of at least 3000 metres, the underlying rock has not
vet been found.
The principal rocks of which Iceland is composed are two,
basalt and palagonite breccia; more than one-half of the surfaee
and the rock-foundation of the country consists of basalt, but the
palagonite formation, which is composed of breccias, tuffs and
conglomerates of different age and which, taken as a whole, is
vounger than the basalt formation, forms an irregular band across
the country, occupying an area somewhat smaller than that occu-
pied by the basalt. Compared with these two formations, all other
rocks and formations have quite an unimportant distribution. The
basalt mountains, the precipitous walls of which often rise from
the sea to a height of 600-1000 metres, are composed of Iayers of
varying thickness, wedged in between each other; the thickness of
the individual layers often decreases rapidly along a short distance
until the layer disappears and gives place to another. In the basalt
formation, beds of tuff and breccia sometimes occur between the
basalt layers, but their amount is inconsiderable compared with that
of the basalt. Dj'kes are frequent; the majority of them pierce down
through the entire series of layers. Seen from a distance, the basalt-
mountains with their steep, terraced walls, have a monotonous and
gloomy appearance, but on closer inspection exhibit rather great
variation. Some of the basalt layers are compact, others are coarsely
crystalline, doleritic, porphyritic, amvgdaloidal (with more or less
completely filled vesicular cavilies), slaggy, banded, etc. In some
districts the basalt is cleft into beautiful columns; in others into
more or less irregular, angular blocks; in others it has an almost
slaty appearance. In the vesicular cavities of the basalt zeolites,
quartz, chalcedony and calc-spar are often found. As a general rule
the basalt layers have a slight inclination (3°—5°) from the coast
inwards towards the tuff and breccia formations, which appear to
fill a flat, saucer-like depression in the underlying basalt plateau;
but many local deviations occur owing to dislocations and sub-
sidences of larger or smaller areas of the underlying rock.
In the middle of the basalt-formation in Iceland (as also in
the Færöes and in Ireland) rather considerable clay deposits are
found with the impressions and remains of plants of Tertiary times;
also lignite and compressed tree-trunks, all called in Icelandic “sur-
tarbrandur.” This plant-bearing formation attains its greatest tliick-