These are a few typical examples of the more violent convulsions by which the Crust of the Earth has been disturbed within little more than a century; and they leave no doubt as to the kind of changes which may fairly be ascribed to similar agency in the past history of the Globe. Nor must it be supposed that, because our examples are few in number, the Earthquake is itself a rare and exceptional event. On the contrary, the state of partial disturbance and convulsion would seem to be the natural and ordinary condition of our planet. From the interesting Catalogue drawn up by Mr. Mallet, it appears that, in our own times, the number of Earthquakes actually observed and recorded is, on an average, not less than from two to three every week. Now this catalogue cannot represent more than one-third of the Globe: for the disturbances which take place in the profound depths of the ocean must for the most part escape observation, and many parts even of the inhabited Earth are still beyond the reach of scientific researches. It is, therefore, quite a reasonable speculation of Sir Charles Lyell, that “scarcely a day passes without one or more shocks being experienced in some part of the Globe.” Moreover, in Mr. Mallet’s Catalogue no account is taken of those minor vibrations or tremblings of the Earth’s Crust, which are not attended by any striking or noteworthy event. And yet such phenomena, when often repeated, may produce a very important change of level, and are far more frequent than most persons would be likely to suppose. In our quiet region of the Globe people are too apt to take for granted the general stability of the Earth: but in other countries the inhabitants, warned by long experience, are no less deeply impressed with a conviction of its instability. Sir John Herschel says that, in the volcanic regions of Central and Southern America, “the inhabitants no more think of counting Earthquake shocks, than we do of counting showers of rain:” nay, he adds that, “in some places along the coast a shower is a greater variety.” And in Sicily, we are told they make provision against movements of the Earth’s Crust, just as we make provision against lightning and storms; so much so that it is quite a common thing for architects to advertise their houses as Earth-quake-proof. [Illustration] [Illustration] _CHAPTER XVII._ SUBTERRANEAN HEAT--ITS POWERS ILLUSTRATED BY UNDULATIONS OF THE EARTH’S CRUST. _Gentle movements of the Earth’s Crust within historic times--Roman roads and temples submerged in the bay of Baiæ--Temple of Jupiter Serapis--Singular condition of its columns--Proof of subsidence and subsequent upheaval--Indications of a second subsidence now actually taking place--Gradual upheaval of the coast of Sweden--Summary of the evidence adduced to establish this fact--Subsidence of the Earth’s Crust on the west coast of Greenland--Recapitulation._