62 Dr. Hans Heck — Fissureless Volcanoes. 



torn off from below, have drilled cylindrical holes into the overlying 

 Karroo shales. 



Now one cannot possibly imagine Herdubreid with a circumference 

 of, let me say, about 14 km., measured along the vertical walls, to be 

 a block torn off and shot up from the underlying ground. Not even 

 a slow upward movement can explain all the facts to be observed, as 

 I have shown in a previous paper. The great bulk of the mountain 

 in a similar way makes it very difficult for me to imagine it as some- 

 thing like a parasitic cone in comparison with a great volcano. For 

 where is the great volcano or the grand volcanic fissure of which 

 Herdubreid should be a parasite ? Neither of them is at present 

 known in Iceland. 



If we compare Professor Schwarz's hypothesis of Herdubreid as an 

 offshoot of some unknown volcanic fissure, and Daubree's experiments, 

 ■with the facts observed in nature, we are confronted at once with 

 the question : Do all the leading features really coincide in nature 

 and in the experiment ? In Professor Schwarz's theory as well as in 

 Daubree's experiments gases are the moving factor of the eruption, 

 and their prevailing action is the necessary proposition to the 

 experiment, as it depends mainly on their explosive force. Do we 

 find anything like that in the case of Herdubreid? Observation gives 

 not the least support to the assumption of any explosions worth 

 mentioning, neither in the beginning of the first eruption nor during 

 the course of later ones. 



By climbing up the steep cliffs of palagonite and lava in ascending 

 the top of Herdubreid for the first time I actually passed over the 

 entire section across the mountain, and had to climb from sheet to 

 sheet of the basaltic lava-flows of the upper region. But neither 

 at the base of the lowest one nor as accumulations between later 

 outflows could I find loose volcanic material such as ashes, lapilli, 

 bombs, pumice, or broken pieces of the ground, which are the most 

 evident companions of all volcanic explosions. On the contrary, the 

 want or but feeble display of explosions is one of the most characteristic 

 peculiarities of the ' Masseneruptionen', of fissure eruptions as well as 

 of the eruptions from central craters in the case of lava volcanoes. 

 No ' Schild ' volcano is known in Iceland to have erupted in historic 

 times, but all the characteristics of the crater and its surroundings 

 point to a mechanism of eruption just as we can observe it in the huge 

 lava volcanoes of Hawaii, Kilauea, and Mauna-Loa, where in historic 

 times all the eruptions but one have taken place without the 

 development of any remarkable explosions. Prom none of the many 

 ' Schild ' volcanoes of Iceland is explosive material known, and 

 Herdubreid is no exception to the rule. Therefore I must conclude 

 that not the gases but the magma itself has been the main force 

 which led it to eruption. In the course of long-repeated gentle 

 outflows it piled up the Herdubreid mass. But as no indication 

 of a surplus of gases is recognizable in the erupted materials I think 

 it most improbable that explosions, which would have drilled 

 a cylindrical pipe through the ground — either on a fissure or beside 

 it — took place during the origin of Herdubreid. 



Thus the gradual ascent of lava with but little gas in a pipe must 



