96 SCIENCE PROGRESS 



diameter; it is surrounded by Jurassic strata resting on 

 granite, whilst in the depression, whose floor is the same granite, 

 the level of this rock is above that in the surrounding country, 

 where it is covered with sedimentary beds. The Ries granite, 

 then, is a gigantic plunger which has been elevated by volcanic 

 forces, and the balance of evidence seems to indicate that the 

 fault-blocks of Iceland have been elevated in a similar manner, 

 although they are bounded by quadrilateral and not circular 

 faults. 



The simplest example of a volcano of block-uplift is the 

 Herdubreid in the lava desert of the Odadahraun. The cliffs 

 surrounding the block are some i, 800 ft. high, 300 ft. of which 

 are concealed under tabus heaps. The rock comprising them is 

 brown pelagonite tuff, covered on top with the basalt, which 

 flowed from the central chimney. The volcano is of the buckler 

 type, with a deep central crater, from which lava poured out in 

 a symmetrical low angle cone. It is 5,450 ft. high and rises 

 some 4,000 ft. above the plain. The walls of the pedestal on 

 which the lava rests are kept quite fresh by the enormous 

 weathering that goes on in such regions ; there is no sign of 

 any fissure traversing them by which the volcanic gases could 

 have risen to form the chimney. The block has been driven 

 upwards between two sets of crossing faults and an escape 

 vent has been drilled in the centre through the solid rock. 



To the south-west of the Herdubreid lies the much larger 

 Dyngjufjoll block, with the square caldera of Askja at its 

 summit. The lava desert, with its surface so scoriaceous and 

 rent with chasms that it is all but impossible to traverse, is 

 here covered with pumice thrown out by the Rudloff crater 

 which lies in the Askja. A narrow gorge, the Askja Op, leads 

 up to the top at the north-eastern corner. On entering the 

 Askja, one finds oneself in a wide, level plain filled with slaggy 

 lava and surrounded on all sides by steep hills, whose crests 

 turn round at right angles and enclose the square caldera. 

 The area of the depressed lava-field is about sixteen square 

 miles. The surrounding hills rise from it 1,000 to 1,200 ft., 

 but from the outside the}' rise from 2,000 to 2,500 ft. The outer 

 dimensions of the block are, roughly, fifteen miles on all four 

 sides. The remarkable fact about the Askja is that the boundary 

 hills are made of the older pelagonite tuff of the same nature as 

 that forming the pedestal on which the Herdubreid volcano 



