320 



PHYSICAL GEOLOGY 



ever, but are interrupted in many places by areas in which no 

 volcanoes occur. 



Although volcanoes are usually situated along the borders of con- 

 tinents, this is not always the case ; some volcanoes in Ecuador, for 

 example, are 150 miles inland, and in East Africa the volcano Kirunga 

 is 600 miles from the coast. 



The most important of the volcanic belts almost encircles the 

 Pacific Ocean, extending from the southern tip of South America 

 northward along the Andes on the western coast of that continent, 

 through Mexico, and along the western coast of North America to 

 Alaska. From Alaska it curves westward and southward through 

 the Japanese and Philippine archipelagoes to New Zealand and to the 

 Antarctic volcanoes. The borders of the Atlantic, in contrast to those 

 of the Pacific, are almost free from volcanoes. Two important belts, 

 however, occur in this ocean ; one stretches from Iceland south to St. 

 Helena and includes the Azores and other volcanic islands ; the other 

 includes the West Indies and the shores of the Mediterranean Sea. 



Cause of Distribution. — A study of regions of volcanic activity 

 brings out the fact that they have recently undergone severe move- 

 ments, or are actually being deformed at the present time. In other 

 words, volcanoes are situated where mountain-making forces (p. 358) 

 are active, and where, consequently, the earth is much fissured and 

 fractured (p. 360). The fact that belts of active volcanoes are usu- 

 ally found where mountain ranges are near or parallel to great deeps 

 in the neighboring oceans has given rise to the belief that the eleva- 

 tion of the strata of which mountain ranges or islands are composed 

 is compensated by a sinking of the ocean bottom, and that as a result 

 of these movements lava and ash are ejected to form volcanoes. It 

 is to be noted in this connection that volcanic activity tends to die 

 out in the older rocks and to appear in those of later date. 



It is evident from the above that the problem of the distribution 

 3f volcanoes is an important one, since on its solution must depend in 

 x large measure the much more general one of the cause of volcanism. 



Ancient Volcanoes. — The volcanoes of the past had as a rule a 

 different distribution from those of the present. For example, Great 

 Britain and central France were the scenes of intense volcanic activity ; 

 the Connecticut valley, northern New Jersey, and many of the western 

 states (Wyoming, Colorado, New Mexico, Idaho, and others) have ex- 

 perienced great lava flows, or many and great volcanic eruptions. At 

 a much earlier period in the earth's history (Pre-Cambrian) volcan- 



