392 Professor E. H. L. Schwars—Fissure Volcanoes. 
imperforate. At the same time their. front walls are obviously of 
Cribrilinid type, i.e. built up from marginal spines more or less fused 
together, and it seems to me, as indicated at p. 292 of the volume of 
this Magazine for 1906, that in the Chalk it is not feasible to maintain 
one particular stage in the lateral fusion of spines as a separate genus 
Cribrilina, and that that genus must be enlarged to comprise all.stages 
of lateral fusion short of total obliteration of the spinous origin. The 
only alternative would be to create a new genus for all stages of 
lateral fusion except Cribrilina, as usually defined; such a genus would 
not admit of a positive definition, and is obviously undesirable. 
EXPLANATION OF PLATE XXX. 
'Fre. 1. Rhagasostoma Novaki, var. Anglica. Trimingham. x 12 diam. 
5, 2 ‘Oribrilina claviceps. Grayesend, Kent. x 12 diam. 
,, 8 Ditto, the same specimen. x 21 diam. 
,» 4 Ditto. Whitchurch, Hants. x 12 diam. 
»» . de, Ditto, Gravesend. x12diam.. . 
,, 6. Oribrilina furcifera. Andover, Hants. x 12 diam. 
», 7. Ditto. Bramford, Suffolk. x 12 diam. 
;, 8. Ditto.: Kingsgate, Kent.. x: 12 diam. 
5 9. Oribrilina Filliozati. Andover, Hants. x 12 diam. 
», 10. Ditto. West Tytherley, Hants. x 21 diam.. 
(To be continued.) 
' JIJ.—Tue Fissure Turory or VoLcanogs. 
By Professor E. H. L. Scuwanz, A.R.C.S., F.G.S., Rhodes University College, 
Grahamstown. 
R. HANS RECK has adduced an example of a voleano which, 
according to him, has been formed independently of a fissure. 
‘The volcano pierces the centre of a faulted block, the Herdubreid, 
Iceland, and on the vertical fault-faces there is no sign of any fissure.’ 
The example is probably unique in the world, and seems at first sight 
to negative the hypothesis that the escape of gases which tear through 
the earth’s crust and form the chimneys of volcanoes is in the first 
place initiated, by a fracture; on closer examination, however, the 
fact that the volcano stands in close relation to the faults which bound 
the horst, and the many cases which are known to occur where 
a fracture in the earth’s crust may be healed at the surface so that 
the rocks about the fracture are subsequently more resistant than 
before, seem to point to the Herdubreid volcano being a normal 
fissure-formed volcano, only that it stands in the same relation to the ~ 
fracture as a parasitic cone stands to the central crater. In other 
words, the chimney is an escape vent leading below the surface to one 
of the bounding faults of the horst. . 
The Herdubreid is a part of the north and south range of mountains 
made of palagonite tuff, which sinks towards the north below the 
level of the Odadahraun lava plateau, some 600 metres above sea- 
level, and on the south rises to 1077 metres. The Herdubreid ‘is 
1200 metres high, and is separated from the main range by a gorge 
1 Hans Reck, ‘‘ Ein Beitrag zur Spaltenfrage der Vulkane’’: Centralblatt fiir Min., 
Geol., u. Palaeont., 1910, No. 6, p. 166. 
