Professor E. H. L. Schwarz—Fissure Volcanoes. 393 
500 metres wide ; its walls are vertical, but the lower parts are hidden 
by debris. The square summit rises gradually to the crater, with its 
lavas of much more recent date than the palagonite tuff. The sub- 
structure of the volcano is shown in the four bounding fault-faces to 
a height of 700-800 metres above the accumulations of debris. On 
the north side there is a horizontal line where a darker and lighter 
material in the tuff meet; the whole length of the line is exposed, 
* and there is no trace of a dislocation. The material forming the 
basement of the horst is quite fresh on the exposed side, since the 
outer surface is rapidly weathering under the influence of frost, and 
the weathered material is removed by wind; this applies to all the 
four sides of the block, so that from all sides the evidence of the want 
of a fissure is perfectly clear. Dr. Reck concludes his article with the 
statement—It is therefore proved from observation that in one case 
at least a fissure does not extend under the basement of a volcano for 
a depth of from 300 to 400 metres. 
One of the best examples I know where a fracture in the earth’s 
crust has been healed is in the Berg River Hoek, near Paal in Cape 
Colony. I was called in to report on the nature of the valley with 
regard to its being suitable for a large reservoir. The site of the 
proposed retaining wall was traversed by a great fracture, on either 
side of which the rock, quartzite belonging to the Table Mountain 
Series, had been brecciated for 3 or 4 yards. I investigated the 
fracture very fully and found that the crush-breccia, through cementa- 
tion by secondary silica, had not only become very much more compact 
than the quartzite itself, but that the uncrushed rock beyond the 
crush-breccia had been also hardened by the deposition of secondary 
silica. The whole zone in the neighbourhood of the fracture was 
eminently suitable for the foundation of a retaining wall; not only so, 
but the cementing of the neighbouring rock showed that subsequent 
movement would not take place along the old fracture, but would 
have to find relief some distance away. 
Dr. Reck admits that all the volcanic fissures of the North of 
Iceland lie in a north and south direction, parallel to two of the 
bounding faults of the Herdubreid horst, so that it is at least admissible 
to those who believe in the fissure theory of the origin of volcanoes to 
consider the Herdubreid chimney to be an offshoot of a volcanic fissure, 
the surface end of which has been closed by cementation. 
Daubrée’s experiments on the exploding of dynamite in steel shells 
shows us how a fissure may allow the escape of gases, and that these 
in tearing their way to the surface may drill cylindrical holes; the 
minute fracture in the case of the shells need not have extended to the 
surface. I have in mind, however, what I have termed a fossil earth- 
quake, where a spherical mass of dolerite has been shot off from 
a horizontal sheet of that rock and has drilled a hole vertically 
through the overlying shale for a height of 15 feet. The example 
is at Cradock in Cape Colony. The cause for the ejection was 
probably that the molten rock encountered a reservoir of water, and 
in the explosion which followed a portion of the dolerite was projected 
upwards. The space where the projected mass of rock once was, and 
the path of the projectile, are filled with crushed-up shale. I call it 
