chap. 1 
GEOLOGICAL ADVANTAGES OF BRITAIN 
7 
3. In the third place, this advantageous geographical position is found 
to have been attended with an altogether remarkable alnindaiice and per- 
sistence of volcanic eruptions. No tract of equal size yet known on the 
face of the globe furnishes so ample a record of volcanic actirnty from the 
eai’liest geological periods down into Tertiary time. Nvery degree of 
energy may be signalized in that record, from colossal eruptions which piled 
up thousands of feet of rock down to the feeblest discharge of dust and 
stones. Every known type of volcano is represented — great central cones 
like Etna or Vesuviirs, scattered groups of small cones like the puys of 
France, and fissure- or dyke-eruptions like those of recent times in Iceland. 
iloreover, the accurate manner in which the stratigraphy of the country 
has been established permits each successive era in the long volcanic history 
to be precisely determined, and allows us to follow the whole progress of 
that history stage by stage, from the beginning to the end. 
These characteristics may be instructively represented on a map, such 
us that which accompanies the present volume (Map I.). The reader will 
there observe how repeatedly volcanic eruptions have taken place, not 
merely within tlie general area of the British Isles, but even within the 
Same limited region of that area. The broad midland valley of Scotland 
has been especially the theatre for their display. Erom the early part ot 
the Lower Silurian period, through the ages of the Old Bed Sandstone, 
Carboniferous and Permian systems, hundreds of volcanic vents were active 
hi that region, while in long subsequent time there came the fissure- 
uruptions of the Tertiary series. 
d. In the fourth place, the geological revolutions of successive ages 
have made this long volcanic chronicle fully accessible to observation. Had 
tile lavas and ashes of one period remained buried under the sedimentary 
accumulations of the next, their story would have been lost to us. e 
ehould oidy have been able to decipher the latest records which might 
happen to lie on the sui’face. Eortunately for the progress of geology, 
the endless vicissitudes of a continental border have brought up the very 
eldest rocks once more to the surface. All the later lormations of the 
earth’s crust have likewise been upraised and exposed to denudation during 
long cycles of time. In this manner, the rocky framework of the country 
has been laid bare, and each successive chapter of its geological history may 
he satisfactorily deciphered. The singularly complete volcanic chronicle, 
after heing entombed under younger deposits, has been broken up and 
raised once more into view. The active vents of former periods have been 
dissected, submarine streams of lava have been uncovered, sheets of ashes 
that fell over the sea-bottom have been laid bare. The progress of de- 
mxdation is specially favoured in such a variable and moist climate as that 
of Britain, and thus by the co-operation of underground and meteoric causes 
fhe marvellous volcanic records of this country have been laid open in 
minutest detail. 
There is yet another respect in which the volcanic geology of Britain 
possesses a special value. Popixlar imagination has long been prone to see 
