206 The National Geographic Magazine 



from Iceland, from north to south an 

 irregular submerged ridge bears the 

 volcanic mountains of the Azores, the 

 Cape Verde Islands, Ascension, St. 

 Helena, and Tristan da Cunha On 

 the west edge of the Atlantic are the vol- 

 canoes of the West Indies ; but north or 

 south of the Antilles there is not a single 

 volcano on the east coast of America. 

 The volcanic belt of the Mediterranean 

 shore is prolonged to the mountains of 

 Armenia and western Arabia. There 

 are said to be some volcanoes in Tibet 

 and Manchuria, but the explorer has 

 not yet located them. 



Elisee Reclus has drawn attention to 

 the fact that the great centers of vol- 

 canic action in the western and eastern 

 hemispheres are at exactly opposite 

 ends of the globe — are at antipodes to 

 each other — and that these centers of 

 activity are near the poles of flattening. 

 They also flank, one on the west and 

 one on the east, the immense circle 

 around the Pacific. 



Volcanoes may be roughly described 

 as of two types — the expulsive and the 

 explosive. Of the first, Hekla, in Ice- 

 land, Stromboli, and Mauna Loa in the 

 Hawaiian Islands, are good examples. 

 They pour forth masses of lava which 

 flows like molasses. Of the second type 

 are Vesuvius, Mont Pelee, the volcanoes 

 of the West Indies, and those of the 

 Andes and of Mexico ; these eject the 

 material andesite, and are more explo- 

 sive than those ejecting the ropy lavas. 



CAUSES OF VOLCANIC ACTION 



Theories abound as to the cause of 

 volcanic action, but of actual causes we 

 know little. Science has no X rays to 

 pierce into the bowels of the earth. 



Lafcadio Hearn, in one of his inter- 

 esting sketches of the French West 

 Indies, published some years ago, tells 

 the story of perhaps the only man who 

 ever descended into the earth while it 

 quaked. It seems that during a certain 



convulsion that shook and rent a certain 

 island of the West Indies one man was 

 thrown far down a fissure. He was un- 

 harmed, but his position, as he tumbled 

 far down in the fissure, was not such as 

 to inspire hope or a scientific study of 

 what had happened or was happening. 

 After some time passed there in the 

 bowels of the earth, another convulsion 

 shook and rent the ground. It tossed 

 him up and up and out of the fissure and 

 landed him unharmed on solid and firm 

 ground ; but when asked how it had all 

 happened, the process of all this tossing, 

 he could not explain. Science had lost 

 its one chance of learning by personal 

 observation what is happening beneath 

 us. 



Perhaps the most probable explana- 

 tion of. explosive eruptions of volcanoes 

 is as follows : 



The rocks deep beneath the surface 

 are kept moist by the water that slowty 

 seeps through. Probably the rocks 

 contain from 3 to 20 per cent of water. 

 The heat of the molten mass beneath 

 the rocks gradually generates steam, 

 and as time goes on more and more 

 steam is generated. The pressure of 

 this steam is constantly increasing until 

 a time comes, when the weight above 

 can not hold in the expanding force of 

 the steam. Like a boiler, the whole 

 mass explodes with terrific fury. An 

 earthquake ma}' open a fissure which, 

 by letting down water rapidly, will 

 hasten the explosion ; but it is doubtful 

 if an earthquake can do more than this. 

 Water entering b}' a fissure could hardly 

 invade the vast area upheaved by an 

 explosive eruption. 



The Guatemalan earthquake of April 

 probabh' timed the explosion of Mont 

 Pelee and La Soufriere. It was the 

 last straw ; it brought the last ounce 

 of pressure — one ounce more than the 

 boiler could bear. The local earth- 

 quakes in Martinique and St. Vincent 

 were the ruptures and tremors caused 

 by the fettered steam. 



