THE 



GEOLOGICAL MAGAZINE 



NEW SERIES. DECADE VI. VOL. IV. 



So. III.— MARCH, 1917. H J[& 



OIRIGS-IIDT.A.L ABTICLBS. \Q 



-A Remarkable Rock Stream in East Icelan\, $ff n , , 

 By Leonard Hawkes, M.Sc, F.G.S. 

 (PLATE VI AND TEXT-MAP.) 



IN his account of the -volcanoes of Iceland, Thoroddsen describes 

 two types of acid lavas which have been extruded in post- 

 Glacial, prehistoric times. First are the lavas of the Torfajokull 

 district, of the usual nature of acid flows, building up a compact mass 

 of bluish-grey rock with an outer casing of obsidian and pumice. 

 These lavas have been poured out in the post-Tertiary country, but 

 the second type of stream is found most frequently in the older 

 parts of the island, which otherwise have witnessed no volcanic 

 eruptions since Tertiary times. This type of flow results from 

 eruptions of a peculiar character in which "vast outflows of half- 

 melted and unmelted masses of liparite, poured out from cauldron- 

 shaped depressions, stretch down into the lowlands " (1, p. 503). 

 The best example of these " Liparitische Blockstrome " occurs in the 

 Lodmundarfjord district, E. Iceland, and is especially described by 

 Thoroddsen (2, pp. 159-161). In this paper the contention is put 

 forward that the Lodmundarfjord blockstream is not a lava-flow but 

 an unusual type of glacial moraine. 



Unfortunately the magnificent topographical map of Iceland in 

 preparation by the " Generalstaben " of Copenhagen does not yet 

 include the Lodmundarfjord district, and no claim of special accuracy 

 is made for the rough sketch-map of Fig. 3, which is given to render 

 the description below more intelligible. The Lodmundarfjord, 

 running approximately east and west, is bounded on both sides by 

 ramparts of Tertiary plateau-basalts from two to three thousand feet 

 in height. The fjord is continued inland in a broad flat valley, the 

 Bardarstadadal, and a raised beach, 23 metres high, after Thoroddsen 

 (2, p. 103), shows that the sea formerly stretched far up the valley. 

 The only notable breach in the southern wall occurs at 800 feet, the 

 mouth of a broad hanging valley which forms part of the pass 

 (the HjalmadalsherSi, circa 2,000 feet) leading over to the Seydisfjord. 

 On the north side, near the end of the fjord proper, a broad valley, 

 the Hraundal, leads up from sea-level to the inland plateau in 

 a north-westerly direction. The mountains of the Lodmundarfjord 

 are built up exclusively of basalts and red tufl partings, but a thick 

 series of acid rocks is exposed in the cliffs bordering the upper reaches 

 of the Hraundal, being the southern extension of the largest area of 



DECADE VI. — VOL. IV. — NO. III. 7 



