IGNEOUS AGENCIES. 



143 



and height of the original volcano before ingulfment. 

 This former volcano has been named Mt. Mazama. 





FIG. 7(i. Ideal section of Crater Lake, Mt. Ma/auia, and Wizard Isle. (After Diller.) 



Age of Volcanoes. From the progressive manner in 

 which volcanoes grow, it would seem that we may esti- 

 mate their age. Such estimates, it is true, must be very 

 rough, yet they are useful in familiarizing the mind with 

 the idea of the great amount of time necessary to account 

 for geological phenomena. For this purpose we will use 

 Etna. There have been, indeed, other volcanic eruptions 

 great enough to build this mountain at once^but the 

 eruptions of Etna itself have been very regular and mod- 

 erate. 



Etna is 11,000 feet high, and about thirty miles in di- 

 ameter at its base. We will take its circumference at one 

 hundred miles. Now, a lava-stream of triangular shape, 

 one foot thick, reaching to the base, and one mile wide, 

 would, we believe, be an average eruption. It would 

 cover seven square miles, one foot deep, and would be 

 equal to more than 200,000,000 cubic feet. It would 

 take one hundred such eruptions to raise the whole 

 mountain-surface one foot. Taking one such eruption 

 every year (eruptions of Etna for the last 2,000 years have 

 been but one in twenty-five years), it would take a century 

 to raise the mountain-surface one foot. But there is a 

 gorge cut into the side of this mountain, revealing 3,000 

 feet of lava-layers. To have built up these 3,000 feet 

 would require 300,000 years. That we have been moder- 



