262 
THE TERTIARY VOLCANOES 
BOOK VIII 
cited iu almost every handbook of geology as types of modern volcanoes and 
their operations. The regular volcanic cone, composed ol alternations ol 
lavas and tuffs, plays a very subordinate part in Iceland. 
The fundamental feature in tire Icelandic eruptions is the production of 
fissures which reach the surface and discharge streams ol lava from many 
points. Two systems of such fissures appear to be specially marked, one 
in southern Iceland running from south-west to north-east, the other, in 
the north part of the island, stretching from south to north. 1 Hekla 
and Laki belong to the former. The dislocations have often followed the 
boundaries of the “ horsts ” or solid blocks of country which have withstood 
Fia. 292. — Fissure (gja) in a lava-field, Iceland. (From a photograph by Dr. Tempest Anderson.) 
terrestrial displacement. The vast outbreaks of Odadahraun and Myvatn 
have almost all issued from fissures of that nature. 
The violent eruption of 1875 in Askja found its exit at the intersection 
of two lines of fissures. Many large fissures were opened on the surface in 
a nearly north and south direction, which could be followed for 80 kilo- 
metres or nearly 50 English miles. Some of them became the theatre 
of intense volcanic activity. 2 
Many lines of fissure are traceable at the surface as clefts or “ gjas,” that 
run nearly straight for long distances, with a width of one to three yards, 
and sometimes of unknown depth. 3 The most stupendous example of the 
structure yet discovered is probably the Eldgja found by Dr. Thoroddsen in ? 
the year 1893, below the M^rdalsjbkull. This gigantic chasm has a length 
1 In the Snaefell promontory they run nearly east and west. Mr. Thoroddsen, Bihang. Svensk. 
Akad. xvii. (ii.) No. 2, p. 91. 
2 Mr. Thoroddsen, op. tit. xiv. ii. No. 5, p. 63. 
3 On the various modes of origin of these chasms, see Dr. Tempest Anderson, Brit. Assoc. Hep. 
p. 650. The gja shown in Fig. 292 is not an eruptive fissure. For this and the following illus- 
tration I am indebted to the kindness of Dr. Tempest Anderson, who himself photographed 
the scenes. 
