I9I0.] INLAND-ICE OF THE ARCTIC REGIONS. 65 



them further from mountain glaciers. As was found true of the 

 Norwegian glaciers, so here the tongue which follows the valley 

 bottom and which partakes of many of the properties of a mountain 

 glacier, is much the narrower.^ 



From the north or plateau margin of the VatnajokuU flow 

 mighty but sluggish streams which near the glacier are braided into 

 constantly shifting channels within a broad zone of quicksand. In 

 this sand, horse and rider if once entangled are quickly lost. Upon 

 the south margin, on the other hand, the streams from the melt- 



-iOOO — 

 -1500 — 

 • 1000 — 



- soo — 



Fig. 5. Cross section of the Vatna Jokull from north to south (after 

 Thoroddsen and Spethmann). 



ing of the ice flow as fast rushing rivers, sometimes so broad as 

 not to be bridged, and in these cases setting up impassable bar- 

 riers between districts. 



Icelandic ice-caps differ from all well-known glaciers at least 

 in this, that nowhere else are large ice masses in such direct asso- 

 ciation with so active volcanoes. The jokulhlaup, which is the Ice- 

 landic name applied to one of the characteristic catastrophies of the 

 island, occurs whenever a volcano, either visible in the neighbor- 

 hood of the glacier or hidden beneath it, breaks suddenly into 

 eruption. The first intimation that such an event is transpiring, is 

 often the drying up of a stream which flows from the affected 

 region. Sometimes the people are permitted to see great masses of 

 lava and volcanic ash issue together from the glacier. All at once, 

 the stream which had first dried up comes rushing down its valley as 

 a foaming flood of water, spreading out for miles and having a depth 

 sometimes as great as 100 feet. The entire plain is then spread 

 with mud and sown with great rocks and also with ice blocks, some 

 of which are as large as the native houses. These ice blocks are 



" Carl Sapper, " Bemerkungen iiber einige siidislandische Gletscher," 

 Zcitsch. f. Gletsch., Vol. 3, 1909, pp. 297-305, two maps and three figures. 

 See especially Fig. 3. 



PROC. AMER. PHIL. SOC., XI.IX, I94 E, PRINTED JUNE 9, I9IO. 



