110 GEOLOGY. 



Fig. 44, b. 



when, by volcanic action. There is -a group of such isl- 

 ands in the Grecian Archipelago, the advent of some of 

 which is indeed known. A new island was thrown up 

 near Iceland in 1783, consisting of high cliffs, and from 

 various parts of it there were emitted fire, smoke, and 

 pumice. This island was taken formal possession of by 

 his Danish majesty, but before a year had passed there 

 was nothing left to show where it was but a reef of 

 rocks. 



200. Earthquakes. As earthquakes very generally 

 precede an eruption of a volcano, and cease when the 

 lava pours forth, the conclusion is legitimately arrived at 

 that all earthquakes are caused principally by the move- 

 ments of pent-up volcanic matter. When they occur at 

 a considerable distance from volcanoes, they are owing 

 to heavings of that vast body of melted matter which is 

 contained in the interior of the earth ; and it has been 

 observed in such cases that, when there has been a suc- 

 cession of earthquakes, they have ceased when some vol- 

 cano has, by its eruption, relieved the pressure, or when 

 some earthquake of great violence has had the same 

 effect. Thus, in 1811, there were many earthquakes in 

 South Carolina, which ceased altogether when Caraccas 



