There may be local tremors due to causes which are less immense and world-wide. One such cause might be the collapse of cavities in the earth. We are well acquainted with some such caves near the surface of the earth. These caves, especially in limestone, are commonly caused by the action of springs. Even pure water will dissolve a minute quantity of the substance of many rocks, and rain water is far from being chemically pure water. It takes oxygen and carbonic acid out of the air as it falls, and it abstracts acids out of the soil through which it sinks. The presence of this acid gives the water a greatly increased power of attacking carbonate of lime. Now limestone is a rock almost entirely composed of carbonate of lime. It occurs in most parts of the world, covering sometimes tracts of hundreds or thousands of square miles, and often rising into groups of hills and ranges of mountains. The abundance of this rock offers ample opportunity for the display of the dissolving action of subterranean water. The water trickles down the vertical fissures along the planes below the limestone beds. As it flows on it dissolves and removes the stone till in the course of centuries these passages are gradually enlarged into clefts, tunnels, and caverns. The ground becomes honeycombed with dark subterranean chambers, and running streams fall into these chambers and continue their course underground. In England there are famous "pot-hole" caverns in Yorkshire and the west of England. The Peak Cavern in Derbyshire is believed to be 1200 feet long, and in some places 120 feet high. The caverns of Adelsberg near Trieste have been explored to a distance of several miles. The River Poik has broken into one part of the labyrinth of chambers through which it rushes before emerging again to the light. "Narrow tunnels," says Sir Archibald Geikie, "expand into spacious halls, beyond which egress is again afforded by low passages into other lofty recesses. The most stupendous chamber measures 669 feet in length, 630 feet in breadth, and 111 feet in height. From the roof hang white stalactites which uniting with the floor form pillars showing endless varieties of form and size." Still more gigantic is the system of subterranean passages in the Mammoth Cave of Kentucky, the accessible parts of which are believed to have a combined length of about 150 miles. The caverns of Luray, in Virginia, are scarcely less wonderful; and in their case American ingenuity has hit on the idea of sucking the pure, dustless air out of these caverns in order to ventilate a sanatorium. Indeed, a book might easily be written on the wonders of the limestone caverns of the world, but our only purpose in mentioning them in this chapter is to indicate how the rocks of the earth may be made unstable, so that a slight shock may precipitate a catastrophe in them--a kind of subterranean landslip which in its turn may give rise to some of the symptoms of earth tremors.