Iceland / by L. Hawkes [and others].

  • Great Britain. Naval Intelligence Division.
Date:
1942
Catalogue details

Licence: Public Domain Mark

Credit: Iceland / by L. Hawkes [and others]. Source: Wellcome Collection.

    22/582 (page 4)
    or red rock. The total thickness of the series runs into thousands of metres, the base is nowhere seen, and the succession of sheets piled one above another in coastal cliffs and valley sides is one of the most characteristic features (trap-featuring) of the Icelandic landscape. Similar rocks of the same age are found in Greenland, the Faroes, Scotland and Ireland. It is obvious that formerly these basalts ex- tended far beyond the present coasts of these countries, and it is possible that at the time when these lavas were being poured out there was a land bridge from Greenland to Europe. Individual basalt sheets can be traced for many kilometres; they were perhaps poured out from fissure volcanoes (see p. 12). The basalts show all the features exhibited by the present-day lava flows (see p. 8) except that the holes (vesicles) commonly present in the upper and lower parts of the sheets are filled with white chalcedony, zeolites, calcite and other minerals. In the interior of some sheets fractures may be present, normal to their surfaces, which produce a rough columnar structure. The thin beds of brown and red rock so commonly seen between the basalts are weathered volcanic ash or lava or both. They are laterites— rocks containing a high proportion of the oxides of aluminium and iron—and the climate under which they were formed was probably a warmer one than that of Iceland to-day. Beds of brown and dark sandstone and shale up to 50 m. thick also occur, and these contain leaf impressions, fossil wood (surturbrandur) and occasionally thin beds of poor-quality coal (see p. 68). In places, particularly in the east, lavas of acid composition are found between the basalts. These lavas (and ashes), though of small amount in comparison with the basic rocks, are conspicuous by reason of their nature and colour. The lavas commonly have an outer skin of black or green glass (obsidian), whilst their main mass is a fine-grained stone, white, light grey, or pink in colour (rhyolite). Both the glass and the stony rock often show a colour banding, and rounded bodies made up of radiating crystals (spherulites) may be present in such numbers as to form pea or ball-like aggregates. Originally the older rocks formed one great tableland built up of horizontal sheets of rock, but it has been broken up into blocks which ‘have been tilted up to 10° in various directions (Plate 2). Quite com- monly the tilt is directed towards the interior of the country, as may be well seen when sailing in many of the fjords. Erosion has been at work carving deep valleys and eating into the boundaries of the country so that the way in which the rocks were piled up can be studied in the faces of the cliffs. In these cliffs it can be seen that some of the rock