VOLCANIC PHENOMENA. 557 



VOLCANIC PHENOMENA. 



A study of the geological history of England and Wales clearly 

 proves that our country has during several epochs in the past been 

 affected by great volcanic eruptions. The fiery records are pre- 

 served more particularly in the hilly country that lies to the north 

 and west ; and they are to be deciphered from the old lava-flows 

 and ash-beds, and the altering or metamorphism of neighbouring 

 strata. Long ages have elapsed since the last of these eruptions, 

 but as the pleasures of the imagination more often exceed those 

 we realize, so a study of the relics of ancient Vulcanicity amid the 

 mountains of North Wales, the lakes of Cumberland, or the tors 

 of Dartmoor, may yield greater enjoyment than the ascent of some 

 temporarily dormant volcano. 



We must not, however, feel disappointment if we find no distinct 

 craters, for so long a time has elapsed and such changes have taken 

 place since the last eruption affected our land, that the heaps of 

 volcanic material then formed around the vents have been swept 

 away by the agents of denudation. Nevertheless, in some places 

 we have the * basal wrecks ' of old volcanoes, and evidence of the 

 channels through which material escaped to the surface in ' plugs ' 

 of solidified lava. Veins and flows of lava may be traced here and 

 there at all levels and in puzzling relationships to the rocks amid 

 which they lie, and these have, in most cases, been bent and 

 uplifted and weather-beaten, so as to lose all semblance in their 

 present outlines to the volcanic centres of which they once formed 

 part. 



The only modern witnesses we have of phenomena which may 

 be connected more or less directly with the causes of volcanic 

 action are in Thermal Springs like those of Bath, in the evidences 

 of recent upheaval and depression such as are furnished by Raised 

 Beaches and Submerged Forests, and in Earthquakes which now 

 and again, unexpectedly and sometimes painfully, arouse attention 

 to the instability of the Earth's crust. 



The most generally ficcepted view of volcanic phenomena is that they are 

 caused by the percolation of water to the heated interior of the earth, a view 

 supported by the proximity of most volcanoes to the sea-coast, and by the fact that 

 coast-lines are in most cases lines of weakness in the earth's crust. ^ 



A theory propounded by Mr. Mallet was that volcanic action resulted from the 

 heat induced during the contraction of the earth's mass from cooling, that it arose 

 in short from the crushing or shrinkage and fracture of the rocks, leading to much 

 friction and the development of heat.^ The Rev. O. Fisher, on the other hand, 

 regards volcanic energy as the motive power of this contraction. 



' See Prestwich, Proc. Roy. Soc. 1885, p. 253 ; Judd, Volcanoes, ed. 3 ; and 

 works by Scrope, Daubeny, and others. On the subject of subterranean 

 temperature, see Smyth, Address to Geol. Soc. 1868. 



* R. Mallet, Phil. Trans. 1873 ; O. Fisher, Physics of the Earth's Crust; see 

 also Sir J. Herschel, Proc. G. S. ii. 548, 596 ; T. S. Hunt, Q. J. xv. 495. 



