60 Dr. Hans Reck — Fissureless Volcanoes. 



mountains rise abruptly from the plain at their feet to a height 

 of 1,000 to 1,200 metres. Their summit plateau consists of lava 

 gently rising from all sides to the crater. All the mountains in question 

 show exactly the same lines and details of structure, so that it may be 

 just as well here to confine myself to the description of Herdubreid 

 only as a type for all the others. The summit plateau ends abrupti- 

 on all four sides with vertical walls which show a clear and fresh 

 section of the mountain structure, as all the detritus is constantly 

 being removed by the wind, and no vegetation hides even the finest 

 details of the composing rocks. Only the lowest part of Herdubreid 

 up to the height of a few hundred metres is hidden by huge 

 accumulations of debris. 



Beneath the lavas of the upper region follows the main bulk of the 

 mountain, consisting of palagonite tuff. Here it is that one would 

 expect to find a fissure if such existed. It must be stated that all the 

 four mountain walls are practically vertical, uninterrupted, even, and 

 stand at right angles to one another. Two of these lie in the general 

 N.-S. line of all the tectonic and volcanic lines of fissure eruptions 

 in the north of Iceland, but all these faults are of more recent date 

 than the volcano, as parts of it are disturbed by their dislocations. 

 Older faults are up to now not yet known in the surroundings of 

 Herdubreid. 



In the palagonite tuff of the north side of Herdubreid, where we 

 ought first of all to expect to find a volcanic fissure, an horizontal 

 line is to be seen separating a brighter- from a darker- coloured part of 

 the tuff. The uninterrupted course of that line all along the wall 

 absolutely proves that there is at least no line of dislocation on which 

 the volcano could have appeared. Furthermore, the absolutely uniform 

 structure of the tuff, not only here but on all sides, seemed to me 

 proof enough that there was not only no dislocation but just as little 

 a line of weakness or a fissure. A fissure open at the time of the first 

 outbreak of the volcano and no longer open to-day must have been 

 closed since, either by filling up with lava from below or with detritus 

 from above, or, as Professor Schwarz seems to believe, by tectonic 

 movements which, without dislocations, would have pressed together 

 the edges of the fissure. 



The first objection of Professor Schwarz is, that a fissure might 

 exist in the palagonite, but be healed and thus made invisible to the 

 eye. For this he adduces a most instructive example from the Berg 

 River, Hoek, near Paal, in Cape Colony. He has stated that there 

 a great fracture was healed, and the rock brecciated and hardened 

 to a distance of 3 to 4 yards on both sides. Supposing now such 

 a fracture with brecciated and hardened sides would run through 

 the even, vertical, fresh wall of Herdubreid. In the quick process 

 of decomposition of the soft palagonite tuff such a hardened zone 

 of brecciated material would necessarily manifest its character by 

 standing out from the wall as a dyke. On the other hand, it would 

 naturally have to appear as a ravine if the rock along the fault was 

 only brecciated and thus loosened in its structure but not hardened 

 afterwards. But neither the least signs of brecciation nor of . 

 unevenness in the perpendicular mountain side could be observed. 



