Náttúrufræðingurinn - 1958, Blaðsíða 31

LANGISJÓR OG NÁGRENNI
173
to prevent subaerial eruptions even from the highest of the volcanoes. A great
thickness, causing long persistence, of the ice sheet of the last glaciation is
further indicated by an outward direction of the striæ from this area.
After tlie passing away of the last Pleistocene glaciation and throughout
the postglacial climatic optimum the basin of Langisjór probably drained
towards the northeast into the valley of the Skaftá. Later on, an advance
of the Skaftárjökull (a glacier of the Vatnajökull ice-cap) closed this original
outlet of the lake. Possibly, this did not happen until the 14th century.
Then the level of the lake rose to produce the present outlet, called Útfall,
across a col in the ridge of Fögrufjöll. Tliis outlet lias cut a narrow gorge
through the col causing the water level to sink again. Two distinct shore-
lines, 5.5 and 3.5 metres respectively above the present level of the lake,
show that this erosion took place in two successive stages.
About 1890 the front of Skaftárjökull was near its fartliest reach in post-
glacial time, but since then it has receded several hundred metres. Now it is
about to leave the northeastern end of the Fögrufjöll ridge and thereby to
reopen the original passage of draining frorn Langisjór. But this passage and
tlie adjacent end of the lake having been filled with moraine and glacial out-
wash material and the level of the lake having sunk, the outlet of Langisjór
is not likely in the future to return to its original position.
Inguar Hallgrimsson:
Dýrasvifið i sjónum
Eins og kunnugt er, eru plöntur sjávarins hinar einu lífverur
lians, sem rnynda lífrænt efni úr ólífrænum næringarefnum í sjón-
um. Öll dýr sjávarins eru því háð plöntunum, annað hvort eru
þau plöntuætur eða lifa á öðrum dýrum, sem lifa þá aftur á
plöntum.
Dýrum sjávarins má skipta niður í þrjár deildir eða samfélög:
botndýr, sunddýr og svifdýr. Án efa eru svifdýrin sú deild sjávar-
dýra, sem minnst er kunn almenningi. Ástæðurnar eru aðallega
tvær: svifdýrin eru lítið nýtt til manneldis og smæð þeirra er slík,
að fæst þeirra eru greinanleg með berum augum.
Það, sem fyrst og fremst auðkennir svifdýrin frá öðrum dýrum
sjávarins, er að hreyfingar þeirra eru svo kraftlitlar, að þau eru