CHAP. I 
MODERN AND ANCIENT VOLCANOES 
5 
he can best realize how mucli he owes to the process of denudation. 
The volcanic remains of former geological periods have in most cases been 
buried under younger deposits, and have sunk sometimes thousands of feet 
below the level of the sea. They have been dislocated and upheaved again 
during successive commotions of the terrestrial crust, and have at last been 
revealed by the arradual removal of the pile of material under which they 
liad lain. 
Hence we learn that the active volcanoes of the present time, which 
really embrace but a small part of the volcanic history of our planet, are 
the descendants of a long line of ancestors. Their distribution and activity 
should be considered not merely from the evidence they themselves supply, 
ijut in the light derived from a study of that ancestry. It is only when we 
take this broad view of the sidjject that we can be in a position to form 
some adequate conception of the nature and history of volcanoes in the 
geological evolution of the globe. 
In this research it is obvious that the presently active volcano must be 
tbe basis and starting-point of inquiry. At that channel of commuiucation 
between the unknown inside and the familiar outside of our globe, we can 
rvateh what takes place in times of quiescence or of activity. We can 
there study each successive phase of an eruption, measure temperatures, 
photograph passing phenomena, collect gases and vapours, register the fall 
ol ashes or the How of lavas, and gather a vast body ot facts regarding the 
materials that are ejected from the interior, and tlie manner of their 
emission. 
Indispensable as this information is for the comprehension of volcanic 
action, it obviously affords after all but a superficial glimpse of that action. 
We cannot see beyond the bottom of the crater. We cairnot tell anything 
about the subterranean ducts, or how the molten and fragmental materials 
behave in them. All the underground mechanism of volcanoes is neces- 
sarily hidden from our eyes. But much of this concealed structure has 
been revealed in the case of ancient volcanic masses, which have been buried 
and afterwards upraised and laid bare by denudation. 
In yet another important aspect modern volcanoes do not permit us to 
obtain ftdl knowledge of the subject. The terrestrial vents, from which we 
derive our information, by no means represent all the existing points of 
direct connection between the interior and the exterior of the planet. We 
know that some volcanic eruptions occur under the sea, and doubtless vast 
numbers more take place there of which we know nothing. But the con- 
ditions under which these submarine discharges are ellected, the be- 
haviour of the out-flowing lava under a body of oceanic water, and the part 
played by fragmentary materials in tlie explosions, can only be surmised. 
How and then a submarine volcano pushes its summit above the sea-level, 
^nd allows its operations to 1)6 seen, but in so doing it becomes practically 
a terrestrial volcano, and the peculiar submailne phenomena are still effec- 
tually concealed from observation. 
The volcanic records of former geological periods, however, are in large 
