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~ Parr I. Snor. i. § 1.] VOLCANIC PRODUCTS. 197 
Section I. Volcanoes and Volcanic action.’ 
§1. Volcanic Products. 
The term voleanic action (vulcanism or vulcanicity) embraces all 
the phenomena connected with the expulsion of heated materials 
from the interior of the earth to the surface. Among these phe- 
nomena some possess an evanescent character, while others leave 
permanent proofs of their existence. It 1s naturally to the latter 
that the geologist gives chief attention, for itis by their means that 
he can trace former phases of volcanic activity in regions where, for 
many ages, there have been no volcanic eruptions. In the operations 
of existing volcanoes he can observe only superficial manifestations 
of volcanic action. But, examining the rocks of the earth’s crust, 
he discovers that amid the many terrestrial revolutions which 
geology reveals, the very roots of former volcanoes have been laid 
bare, displaying snbterranean phases of vulcanism which could not be 
studied in any modern volcano. Hence an acquaintance only with 
active volcanoes will not afford a complete knowledge of volcanic 
action. It must be supplemented and enlarged by an investigation 
of the traces of ancient volcanoes preserved in the crust of the earth. 
(Book IV. Part VII.) 
The word “ volcano” is applied to a conical hill or mountain, 
(composed mainly or wholly of erupted materials) from the summit, 
and often also from the sides of which hot vapours issue, and ashes 
and streams of molten rock are intermittently expelled. The term 
“volcanic” designates all the phenomena essentially connected with 
one of these channels of communication between the surface and the 
heated interior of the globe. Yet there is good reason to believe 
that the active volcanoes of the present day do not afford by any 
means a complete type of volcanic action. The first effort in the 
formation of a new volcano is to establish a fissure in the earth’s 
erust. A volcano is only one vent or group of vents established 
along the line of such a fissure. But in many parts of the earth, 
alike in the old world and the new, there have been periods in the 
earth’s history when the crust was rent into innumerable fissures 
1 The student is referred to the following works in which the phenomena of vol- 
canoes are specially described. Scrope, “‘ Considerations on Volcanoes,” London, 1825 ; 
“ Volcanoes,” London, 2nd edit. 1872; “Extinct Volcanoes of Central France,’ London, 
1858; ‘On Volcanic Cones and Craters,” Quart. Journ. Geol. Soc. 1859. Daubeny, “ A 
Description of Active and Extinct Volcanoes,” 2nd edit., London, 1858. Darwin, “ Geo- 
logical Observations on Volcanic Islands,” 2nd edit., London, 1876. A. von Humboldt, 
“‘ Ueber den Bau und die Wirkung der Vulkane,” Berlin, 1824. L. von Buch, “ Ueber 
die Natur der vulkanischen Erscheinungen auf den Canarischen Inseln,” Poggend. 
Annalen (1827), ix. x.; “‘ Ueber Erhebungskratere und Vulkane,” Poggend. Annalen 
(1836), xxxvii. EK. A. von Hoff, “ Geschichte der durch Ueberlieferung nachgewiesenen 
natiirlichen Veranderungen der Erdoberflache,” (Part ii., ‘““Vulkane und Erdbeben,”) 
Gotha, 1824. C. W. C. Fuchs, “ Die vulkanischen Erscheinungen der Erde,” Leipzig, 
1865. R. Mallet, “On Volcanic Energy,” Phil. Trans. 1873. HK. Reyer, ‘ Beitrag zur 
Physik der Eruptionen,” Vienna, 1877. Fouqué, “ Santorin et ses éruptions,” Paris, 
1879. References will be found in succeeding pages to other and more special memoirs. 
