CHAPTER XL 
THE MODERN VOLCANOES OF ICELAND AS ILLUSTRATIVE OF THE TERTIARY 
VOLCANIC HISTORY OF NORTH-WESTERN EUROPE 
FROM the facts stated in the foregoing chapters concerning the structure 
of the basalt-plateaux of North-Western Europe, it is evident that in none 
of these areas have the eruptions come from one great central volcano like 
Etna or Vesuvius. On the contrary, in every instance there is abundant 
evidence that the basalt has flowed from many scattered points of erup- 
tion. The uniformity of the lava-sheets in petrographical characters, their 
continuity when viewed in mass, their general horizontally, and their 
constant thinning away in different directions, show that the eruptive 
vents must have been distributed over the whole plateau-areas. 
The conditions under which such eruptions took place can be most 
readily understood by a comparison of the phenomena with those observable 
in modern volcanic tracts where extensive outflows of lava have taken place 
without the existence of any great central cones. Of these regions the 
most instructive is undoubtedly to be found among the recent lava-deserts 
of Iceland. There the parallels to the structures described from the British 
and Faroe plateaux are so numerous and so close that an account of the 
Icelandic region may appropriately be inserted here. 
The evidence furnished by Iceland is of special value in our present 
enquiry, inasmuch as that island, besides its modern eruptions, includes vast 
basaltic plateaux of Tertiary age. These areas of nearly level sheets of 
basalt belong to the same geological period as those of the British and 
Faroe Islands, and display the same internal structure and external features. 
But they have this distinguishing peculiarity that the volcanic fires beneath 
them are not yet extinguished. They have been broken through again and 
again in recent times by volcanic eruptions which have repeated many of 
the characteristics of their Tertiary predecessors. The old and the new 
development of the same volcanic type are thus visible side by side. 
The Tertiary volcanic series of Iceland reaches a thickness of upwards of 
3000 metres, or nearly 10,000 English feet, but as its base is nowhere seen, it 
may be still thicker. Its successive sheets, piled over each other in parallel 
layers, form terraced hills and bold escarpments along the coast, whence they 
slope gently inland. The. plateau, as in the Faroe Islands and in Scotland, 
