170 LIGHT SCIENCE FOR LEISURE HOURS. 



French geologist writing, even so late as 1829, that in 

 ancient times ( tous les phenomenes geologiques se 

 passaient dans des dimensions centuples de celles 

 qu'ils presentent aujourd'hui.' But now we have such 

 certain evidence of the enormous length of the intervals 

 within which volcanic regions assumed their present 

 appearance we have such satisfactory means of de- 

 termining which of the events occurring within those 

 intervals were or were not contemporary that we are 

 safe from the error of assuming that Nature at a single 

 effort fashioned widely extended districts just as we 

 now see them. And accordingly, we have the evidence 

 of one of the most distinguished of living geologists, 

 that there is no volcanic mass ( of ancient date, distinctly 

 referable to a single eruption, which can even rival in 

 volume the matter poured out from Skaptar Jokul in 

 1783.' 



In the volcanic region of which Vesuvius or Somma 

 is the principal vent, we have a remarkable instance of 

 the deceptive nature of that state of rest into which 

 some of the principal volcanoes frequently fall for 

 many centuries together. For how many centuries 

 before the Christian era Vesuvius had been at rest, is 

 not known ; but this is certain, that from the landing 

 of the first Greek colony in Southern Italy, Vesuvius 

 gave no signs of internal activity. It was recognised 

 by Strabo as a volcanic mountain, but Pliny did not 

 include it in the list of active volcanoes. In those 

 days, the mountain presented a very different appear- 

 ance from that which it now exhibits. In place of the 



