328 Revieics — Oeikie's Ancient Volcanoes of Great Britain. 



altered indeed, but the new topography soon ceases to look new. 

 Another generation of inhabitants loses recollection of the old land- 

 marks, and can hardly realize that what has become so familiar to 

 itself differs so much from what was familiar to its fathers. 



" But even when the volcanic covering, thus thrown athwart a 

 wide tract of country, has been concealed under a new growth of soil 

 and vegetation, it still remains a prey to the ceaseless processes of 

 decay and degradation which everywhere affect the surface of the 

 land. No feature of a modern volcano is more impressive than the 

 lesson which it conveys of the reality and potency of this continual 

 waste. The northern slopes of Vesuvius, for example, are trenched 

 with deep ravines, which in the course of centuries have been dug 

 out of the lavas and tuffs of Monte Somma by rain and melted snow. 

 Year by year these chasms are growing deeper and wider, while the 

 ridges between them are becoming narrower. In some cases, indeed, 

 the intervening ridges have been reduced to sharp crests, which are 

 split up and lowered by the unceasing influence of the weather. 

 The slopes of such a volcanic cone have been aptly compared to a 

 half-opened umbrella. It requires little effort of imagination to 

 picture a time, by no means remote in a geological sense, when, 

 unless renovated by the effects of fresh eruptions, the cone will have 

 been so levelled with the surrounding country that the peasants of 

 the future will trail their vines and build their cots over the site of 

 the old volcano, in happy ignorance of what has been the history of 

 the ground beneath their feet. 



" The geologist who undertakes an investigation into the history 

 of volcanic action within the area of the British Isles during past 

 time, with a view to the better comprehension of this department of 

 terrestrial physics, finds himself in a situation of peculiar advantage. 

 Probably no region on the face of the globe is better fitted than 

 these islands to furnish a large and varied body of evidence re- 

 garding the progress of volcanic energy in former ages. This special 

 fitness may be traced to four causes : 1st, the remarkable com- 

 pleteness of the geological record in Britain ; 2nd, the geographical 

 position of the region on the oceanic border of a continent ; 3rd, the 

 singularly ample development to be found there of volcanic rocks 

 belonging to a long succession of geological ages ; and 4:th, the 

 extent to which this full chronicle of volcanic activity has been laid 

 bare by denudation, 



" There is yet another respect in which the volcanic geology of 

 Britain possesses a special value. Popular imagination has long 

 been prone to see signs of volcanic action in the more prominent 

 rocky features of landscape, A bold crag, a deep and precipitous 

 ravine, a chasm in the side of a mountain, have been unhesitatingly 

 set down as proof of volcanic disturbance. Many a cauldron-shaped 

 recess, like the corries of Scotland or the cwras of Wales, has been 

 cited as an actual crater, with its encircling walls still standing 

 almost complete. 



