XII.] EARTHQUAKES AND VOLCANOES. 199 



Graham Island (Fig. 57) appeared in tlie ]\Iediterranean, 

 between Sicily and the coast of Africa, where there had 

 previously been more than 100 fathoms of water. The 

 pile of volcanic matter forming this isle must have been 

 upwards of 800 feet high, for the highest part of the island 

 was 200 feet above water ; while the circumference of the 

 mass of land was nearly three miles. After it had stood 

 above the waves for about three months, the island entirely 

 disappeared. 



It is obvious that a great deal of the force by which 

 volcanic products are brought to the surface is due to the 

 conversion into steam of water which, in some way or other, 

 obtains access to the deep-seated molten rocks; but, whether 

 this is the sole source of volcanic energy or not, is uncertain. 

 Numerous hypotheses have been advanced to explain the 

 source and origin of the molten matter itself. Some of 

 these attempts at explanation refer the heat to chemical 

 and some to mechanical causes ; while others assume that 

 it is merely the residue of the heat which the earth origi- 

 nally possessed, if, as seems likely, it existed at one time in 

 a state of fusion. Dismissing, however, these vexed ques- 

 tions, it is sufficient to remark that some source of heat 

 unquestionably does exist in the earth beneath our feet. 



If a thermometer be buried in the ground at a depth of 

 only a few inches below the surface, it is found to be 

 affected by all superficial changes of temperature, and its 

 indications show that it is cool at night and warm in the 

 day, cold in the winter and hot in the summer. But 

 plunged deep into the ground, or placed in a deep cellar 

 or cavern, these variations disappear, and one uniform 

 temperature is registered under all circumstances. What 

 that temperature is will depend principally on the climate of 



