DYNAMICAL GEOLOGY. 193 



limits himself to a compilation from data recorded in litera- 

 ture, Lyell adds his own observations in confirmation of, or 

 opposition to, received opinions. The geological action of 

 water is first discussed. The destructive and transporting 

 agency of running water is demonstrated by numerous examples, 

 amongst others particular interest attaches to the admirable 

 exposition of the channeling of the Simeto bed at Etna, and 

 the erosion of the Niagara ravine. Lyell, in the earlier editions 

 of this volume, was of opinion that in addition to stream 

 erosion the formation of valleys had been in many cases assisted 

 by the occurrence of earthquakes or landslips, or controlled by 

 local inequalities in the rate of withdrawal of the ocean, but in 

 the later editions he attributed the large majority of valley 

 cuttings to river erosion alone. 



Again, in the earlier editions of this volume, ice and glaciers 

 received little attention, but in later editions a special chapter 

 was devoted to them, and Lyell endeavoured to explain the 

 occurrence of erratic blocks as a result of the transportation of 

 rock-material by icebergs and floes. 



The chapter on volcanoes and earthquakes includes not only 

 a summary of their distribution and manifestations, but also 

 detailed descriptions of the district of Naples and Etna. In 

 describing Monte Somma, and the volcanoes of the Canary 

 Isles and Santorin, Lyell opposes the theory of " Elevation- 

 craters," and explains the circular walls of inclined strata round 

 a central crater as the ruins of former cones of ejected material. 

 In connection with earthquakes, attention is especially directed 

 to the accompanying phenomena of crust-fissures and alterna- 

 tions of level. The variations at the temple of Serapis, 

 near Pozzuoli, are instanced in illustration of the frequency 

 with which changes of level may take place in opposite 

 senses. 



The slower variations of level, independent of volcanism, 

 and affecting large areas, were not fully treated by Lyell in the 

 early editions of the Ptinciples ; but after his travels in 

 Scandinavia, a chapter on this subject was introduced, and in 

 it Lyell supported the view that the northern portion of 

 Scandinavia was slowly rising. 



Lyell attributed volcanoes and earthquakes to the high 

 pressure exerted upon the crust by subterranean vapours and 

 gases which become heated and endeavour to expand. 

 Chemical, electrical, and magnetic influences cause, according 



