VAUX VOLCANIC ROCKS OF ICELAND. 143 
Few, if any, countries present us with such remarkable or various 
manifestations of the potency of subterranean heat as Iceland. So 
much so, indeed, that it appears as though it were the acknowledged 
safety-valve, in this portion of our globe at least, through which the 
troubled contents of depths unknown may, on any emergency, find a 
ready vent. 
Interest more than ordinary appears to have been taken of late in 
this most remarkable region. No less than six parties of tourists 
having, during the year before last, investigated the natural marvels 
of the island. One of these adventurers. Lord Dufferin, in a most 
interesting and amusing book, has given to the world a record of his 
travels. From his " Letters from High Latitudes " we propose to 
quote his graphic picture of the result of volcanic action, in producing 
a very remarkable and very manifest change in the aspect of the large 
tract of country to which that portion of his narrative refers. 
But before we proceed to this, it may perhaps be useful to recal to 
the reader's recollection the two theories which have, up to the present 
time, been put forth with respect to the causes of internal heat, and 
hence to the origin of volcanic action in general- 
Some Geologists are of opinion that the interior of our globe has ever 
been, and still is, in a state of fusion. The result of experiments, 
indicating that in mines the temperature increases gradually with their 
depth, has led to the idea that in the same ratio the temperature must 
increase the nearer wo approach the centre of the earth . Assuming 
this to be the case, it is calculated that at a depth of fifty miles from 
the surface the heat would be about double that of molten iron ; and the 
natural difficulty which arises in consequence is this : What would become 
of the crust of the earth if it confined within it such a mass of fused 
matter as would be the case if this were true ? Other scientific men 
would rather refer these volcanic disturbances to heat generated by 
ever-varying chemical changes which are taking place in the bowels of 
the earth. This theory also is by no means free from difficulties ; 
and it is probable that, in its present state, science is incapable of 
fully drawing aside the veil, and revealing to our view the true 
solution of the problem. 
"We will now, without more ado, introduce our readers to Lord Duf- 
ferin, and beg them to imagine his party landed at Reykjavik, a sea- 
