PHYSICAL GEOGRAPHY 2 If) 



covered on the exterior with a crust of tachvlvte. The tuff-formation 



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appears to consist of several divisions the mutual relation of which 

 has not, however, yet been elucidated. 



All round Iceland, in both the basalt and the tuft' formations, 

 small patches of liparite occur. This rock occurs in small intrusive 

 beds and dykes which, on account of their light colour, are dis- 

 tinguishable from the dark basalt and can therefore often be seen 

 from a distance. The liparites vary very greatly both in colour and 

 in structure, and in places where larger sections are exposed, the 

 colouring is often richly variegated. Liparites are almost always 

 accompanied by many closely allied, glassy rocks, especially pitch- 

 stones, which occur as dykes, perlites, sphaerulites, obsidian and 

 pumice. In south-east Iceland and on Snsefellsnes, veins of grano- 

 phyre are found in some places. The rocky promontories on each 

 side of the Bay of Lon in south-east Iceland consist of gabbro, 

 probably of Tertiary age; this rock is also found under the neves 

 of Vatnajokull, because many of the glacier-rivers carry down an 

 abundance of pebbles of gabbro. The liparites and allied rocks, 

 which on the whole cover an area of about 800 square km., are 

 distributed all over the island, but most frequently in larger quan- 

 tities in East Iceland. Liparite eruptions have taken place at all 

 periods from the earlier Tertiary times to the present day; some 

 volcanoes which have been active within historic times have ejected 

 liparitic pumice. In the neighbourhood of Husavik in North Iceland 

 are found, near Hallbjarnarstadir, marine deposits with abundant 

 shells of mussels and snails dating from the end of the Tertiary 

 epoch, from the period called in England the Red Crag; these for- 

 mations are found nowhere else in Iceland. 



In central Iceland, where tuff and breccia form the foundation, 

 large areas are covered by old ice-striated lava-streams. These lava- 

 streams are distinguished from the lavas of the present day by 

 their colour and structure. The modern lavas are usually dark in 

 colour with a compact basaltic structure, while these ice-striated 

 lavas have lighter colours and a doleritic structure. These dolerite- 

 lavas, during the Glacial period and immediately after, flowed from 

 dome-shaped lava-cones with large crater-openings, here and there 

 still extant; or sometimes from large "Bedded Volcanoes" (Strato- 

 Vulkane) the ruins of which half destroyed by erosion - - are still 

 to be found here and there within the area of the palagonite forma- 

 tion. These ice striated lavas are of different ages; some of them 



