rust is intersected. These
shocks give rise to earthquake waves, and as the crushing of the walls
of the fissure developes heat, we have here the _vera causa_ both of
volcanic eruptions and earthquake shocks--the former intensified into
explosions by access of water through the fissures.--"On the Dynamics of
Earthquakes," _Trans. Roy. Irish Acad._, vol. xxi.

[2] Illustration of the mode of propagation of earthquake shocks will be
found in Lyell's _Principles of Geology_, vol. ii. p. 136, or in the
author's _Physiography_, p. 76, after Hopkins.

[3] "Rep. on Theories of Elevation and Earthquakes," _Brit. Ass. Rep._
1847, p. 33. In the map prepared by Prof. J. Milne and Mr. W. K. Burton
to show the range of the great earthquake of Japan (1891), similar
isoseismal lines are laid down.

[4] Lyell, _loc. cit._, p. 163. Two Catalogues of Earthquakes have been
drawn up by Prof. O'Reilly, and are published in the _Trans. Roy. Irish
Academy_, vol. xxviii. (1884 and 1886).

[5] _Ninth Annual Report, U.S. Geological Survey_ (1888).

[6] _A True and Particular Account of the Dreadful Earthquake_, 2nd
edit. The original published at Lima by command of the Viceroy. London,
1748. Translated from the Spanish.

[7] I take the account from that of Capt. Dutton above cited, p. 220.

[8] Dutton, _Report_, Plate xxvi., p. 308.

[9] _Ibid._, p. 211. On the connection between the moon's position and
earthquake shocks, see Mr. Richardson's paper on Scottish earthquakes,
_Trans. Edin. Geol. Soc._, vol. vi. p. 194 (1892).

PART VII.

VOLCANIC AND SEISMIC PROBLEMS.

CHAPTER I.

THE ULTIMATE CAUSE OF VOLCANIC ACTION.

Volcanic phenomena are the outward manifestations of forces deep-seated
beneath the crust of the globe; and in seeking for the causes of such
phenomena we must be guided by observation of their nature and mode of
action. The universality of these phenomena all over the surface of our
globe, in past or present times, indicates the existence of a general
cause beneath the crust. It is true that there are to be found large
tracts from which volcanic rocks (except those of great geological
antiquity) are absent, such as Central Russia, the Nubian Desert, and
the Central States of North America; but such absence by no means
implies the non-existence of the forces which give rise to volcanic
action beneath those regions, but only that the forces have not been
sufficiently powerful to overcome the resistance o