VOLCANOES AND IGNEOUS INTRUSIONS 321 



ism appears to have been widespread in eastern and central Canada, 

 and large areas in Wisconsin and Minnesota are underlain chiefly 

 with volcanic rock. Throughout geologic history periods of unusual 

 volcanism have been followed by others of comparative quiet. The 

 last important period of volcanism preceded the advent of the Great 

 Ice Age (p. 643), and it is possible that we are now living in the de- 

 clining phases of the activity of that time. 



Importance of Volcanism to Man 



(1) Beneficial Effects. Volcanic regions are interesting not only 

 Because of the striking character of their phenomena and scenery, but 

 also because of their economic value. Abundant springs are usually 

 found in the neighborhood of volcanoes. The ashes from recent erup- 

 tions often form a fertile and easily worked soil. When the surfaces 

 of flows composed of dark-colored lavas (basic, p. 329) are decomposed, 

 they furnish a soil which contains all of the elements needful for 

 plant life, many of which are lacking in granite and other soils ; in 

 Central America, certain regions are benefited far more than they 

 are injured by the showers of volcanic ash, because of the increased 

 fertility resulting from the minerals which these contain. Vesuvius 

 is surrounded by a ring of villages in spite of the danger of eruptions, 

 and the flanks of Etna support an extremely dense population. 



Of benefit to man ; also, are the many lakes that rest in the craters 

 of inactive volcanoes. The lakes of the Alban Hills near Rome, as 

 well as Lake Bracciano and other lakes of Italy, are crater lakes. 

 Crater Lake in Oregon (p. 309) is also a famous example. Lakes 

 have also been formed by the damming of river valleys by lava 

 streams. At the foot of Mt. Shasta, California, are rich tracts of 

 alluvium, the sites of lakes formed in this way and later filled by de- 

 posits which now constitute rich agricultural land. 



Many important ore deposits (p. 371) have resulted from the 

 intrusion of molten rock. 



(2) Harmful Effects. Although volcanoes are sometimes indi- 

 rectly beneficial to man, this does not compensate for the destruc- 

 tion of life and property which result from an eruption. In addition 

 to the destruction wrought by the fall of ash and the outpouring of 

 lava, great disaster has been caused in other ways. Many times in 

 the past, great floods have been brought about by the discharge of 

 the water from lakes which rested in craters, and by the melting of 



