In rotary earthquakes, as the name indicates, the ground is whirled or twisted in the manner of a violent eddy, and is often left in this twisted condition. In the great Calabria earthquake, huge blocks of stone forming obelisks were twisted on one another in a manner represented in Fig. 39. In this case the pedestals remained unaffected, but the separate blocks of stone were partially turned around, as shown. During this earthquake the earth was so twisted that trees, which had been planted in straight lines before the earthquake, were left standing in zigzags. During the great Charleston earthquake, South Carolina, the chimney-tops of the houses were separated at places where they joined the roof and were twisted around these places without being overthrown. In some of the houses wardrobes or bureaus were turned at right angles to their former positions, and in some cases were even found with their faces turned towards the wall. [Illustration: FIG. 39. HEAVY STONE OBELISKS TWISTED BY CALABRIAN EARTHQUAKE OF 1783] Mallet suggests that in some cases the rotary motion is more apparent than real, being due only to a to-and-fro motion without any twisting, the apparent turning being due to the greater freedom of motion of the object in one direction than in another. A twisting motion, however, has actually taken place in some earthquakes. While separate shocks, in a given locality, may follow one another at intervals for fairly long times, yet the principal shock or shake that produces the greatest damage is generally of exceedingly short duration. In the Caracas earthquake the greatest destruction was accomplished in about one minute. There were three distinct shocks, each of which lasted but three or four seconds. The great Calabria earthquake, of 1783, lasted but two minutes. The earthquake of Lisbon, in 1755, lasted five minutes, but the first, and worst, shock, was only from five to six seconds. CHAPTER XXIV SOME OF THE PHENOMENA OF EARTHQUAKES The nature of an earthquake and the movements of its waves from their starting place having now been briefly described, it remains to explain some of the strange phenomena that precede, accompany, or follow one. Next to the violent shaking of the earth's crust, perhaps the most wonderful and impressive thing is the great variety of sounds and noises. These occur not only while the earth-waves are passing through the crust at any place, but also long before the principal shocks reach the place, as well as long after they have passed.