120 



IGNEOUS AGENCIES. 



Vorticose Earthquakes. — In these cases the ground is twisted or 

 whirled round and back, or sometimes ruptured and left in a twisted 

 condition. The most conspicuous examples of this kind of motion 

 occurred in the earthquake of Eiobamba, and in the great Calabrian 

 earthquake of 1783. In this latter earthquake the blocks of stone 

 forming obelisks tuere twisted one on another ; the earth was broken 

 and twisted, so that straight voids of trees were left in interrupted zig- 

 zags. Phenomena similar to some of these were observed also in the 

 California earthquake of 1868. Chimney-tops were separated at their 

 junction with roofs, and twisted around without overthrow ; wardrobes 

 and bureaus turned about at right angles to the wall, or even with 

 their faces to the wall. 



Explanation. — Some of these effects — such as twisting of obelisks 

 and chimney-tops, and turning about of bureaus, etc. — may be ex- 

 plained, as Lyell has shown, without any twisting motion of the earth 

 at all, or any other than the backward-and-forward motion common to 

 all earthquakes. Thus, if we place one brick on another, and shake 

 them back and forth, holding only the lower one, they are almost cer- 

 tain to be left twisted one on the other. The reason is, that the adhe- 



Fig. 102.— Diagram illustrating Reflection of Waves — Map View. 



sion is almost certain to be greater toward one end than the other — the 

 center of friction does not coincide with the center of gravity. This 

 is the probable explanation of twisted obelisks and chimney-tops, etc. 

 Also, the simple back-ancl-forth shaking of a wardrobe in a diagonal 

 direction, would almost certainly lift up one end and swing it around. 



