_ Panr'. Sscr. i. § 4.] VOLCANIC DISTRIBUTION, 259 
as 4, Geographical and geological distribution of 
volcanoes. 
Adequately to trace the distribution of volcanic action over the 
globe account ought to be taken of dormant and extinct volcanoss, 
likewise of the proofs of voleanic outbreaks during earlier geological 
periods. When this is done we learn on the one hand that in- 
numerable districts have been the scene of prolonged volcanic 
activity, where there is now no token of underground commotion, and 
on the other that volcanic outbursts have been apt to take place 
again and again after wide intervals on the same ground, some 
modern active volcanoes being thus the descendants and repre- 
sentatives of older ones. Some of the facts regarding former volcanic 
action have been already stated. Others will be given in Book IV. 
Part VII. | 
Confining attention to vents now active, the chief facts re- 
garding their distribution over the globe may be thus summarised. 
(1.) Volcanoes: occur along the margins of the ocean basins, 
particularly along lines of dominant mountain ranges, which either 
form part of the mainland of the continents or extend as 
adjacent lines of islands. The vast hollow of the Pacific is girdled 
with a wide ring of volcanic foci. (2.) Volcanoes rise as a striking 
feature in the heart of the ocean basins. Most of the oceanic 
islands are volcanic. The scattered coral islands have in all likeli- 
hood been built upon the tops of submarine volcanic cones. (38.) Vol- 
_ canoes are situated, as a rule, close to the sea. When they occur 
inland they sometimes appear in the neighbourhood of a lake. Yet 
as instances have been observed where volcanoes have appeared at 
great distances from any sheet of water, the proximity of a lake or 
of the sea cannot be regarded as always necessary for the evolution 
of volcanic phenomena. (4.) The dominant arrangement of volcanoes 
Is in series along subterranean lines of weakness, as in the chain of 
the Andes, the Aleutian Islands, and the Malay Archipelago. A 
remarkable zone of voleanic vents girdles the globe from Central 
America eastward by the Azores and Canary Islands to the 
_ Mediterranean, thence to the Red Sea, and through the chains 
of islands from the south of Asia to New Zealand and the heart of 
the Pacific. (5.) Onasmaller scale the linear arrangement gives 
place to one in groups, as in Italy, Iceland, and the volcanic islands 
of the great oceans. 
_ Besides the existence of what are called extinct volcanoes, the 
geologist can adduce proofs of the former presence of active volcanoes 
in many countries where cones, craters and all the ordinary aspects 
of volcanic mountains, have long disappeared. Sheets of lava, beds 
of tuff, dykes, and necks representing the sites of volcanic vents 
have been recognized abundantly (Book LV. Part VII.). These mani- 
_ festations of volcanic action, moreover, have as wide a range in 
- geological time as they have in geographical area. ees oreat 
s 
