Prestwich, a well-known English geologist, speaks very decidedly concerning the causes of earthquakes:
"For my own part, I am disposed to share the belief expressed by Dana that the tension or pressure, by which the great oscillations or plications of the earth's crust have been produced, have not entirely ceased; and that this is generally the most probable cause of earthquakes. The uplifting of the great continental tracts and mountain ranges must have always left the interior of the crust in a state of unstable equilibrium, and any slight slide or settling along an old fracture, or in highly disturbed and distorted strata, would be attended by an earthquake shock.
"In volcanic areas the removal of the large volumes of molten rock from the interior to the surface must produce settlements and strains which might also result in some of these minor earthquakes to which volcanic districts are so subject. Where we have the two conditions combined, as they are in the Andes in South America, these earthquake phenomena are, as we should expect, developed on the grandest and widest scale."
Geikie, the Scotch geologist, says:
"Various conceivable causes may, at different times and under different conditions, communicate a shock to the subterranean regions. Such as the sudden flashing into steam of water in the spherodial state, the sudden condensation of steam, the explosion of a volcanic outpour, the falling in of the roof of a subterranean cavity, or the sudden snap of deep-seated rocks subjected to prolonged and intense stress."
Sir Charles Lyell, the great English geologist, holds the following views concerning the origin of earthquakes. He speaks as follows in his "Principles of Geology":
"1. The primary cause of the volcanoes and the earthquakes are to a great extent the same, and connected with the development of heat and chemical action at various depths in the interior of the globe.
"2. Volcanic heat has been supposed by many to be the result of the high temperature which belonged to the whole planet when it was in a state of igneous fusion, a temperature which they suppose to have been always diminishing and still to continue to diminish by radiation into space....
"The powerful agency of steam or aqueous vapor in volcanic eruptions leads us to compare its power of propelling lava to the surface with that which it exerts in driving up water in the pipe of an Icelandic geyser. Various gases also, rendered liquid by pressure at great depths, may aid in causing volcanic outbursts and in fissuring and convulsing the rocks during earthquakes."
Major Clarence Edward Dutton, U. S. A., an acknowledged authority on seismology, speaks as follows: