26o C. E. BUTTON 



2. The second fact is the comparative smallness of the extra- 

 vasated masses in any single volcanic eruption. In order to obtain 

 an idea of the relative magnitude of an erupted mass, let us draw 

 upon a true scale, a segment of i° of the earth's surface, of an arbi- 

 trary thickness — say thirty miles. Upon this segment draw the profile 

 of Vesuvius. About a mile below the surface, beneath the volcano, 

 draw the reservoir of lava, having the same mass as the volcano 

 itself. It may have any thickness and any form, and is subject only 

 to the condition that the capacity of it is the same as the mass of the 

 erupted material. Now, Vesuvius is built of I know not how many 

 individual eruptions, but let us say one hundred; though I presume 

 that there were, in reality, very many more. A single average erup- 

 tion would be the hundredth part of the volume of this reservoir. 

 But there are eruptions known which are many times greater than 

 the average of those of Vesuvius. The largest known in the United 

 States are in the Snake River valley, and while we are not in a 

 position to compute with accuracy their dimensions, we can say with 

 confidence that the volume of the largest of them does not exceed two 

 cubic miles. The great eruption of the Skaptar Jokul, in Iceland, 

 in the year 1783, was estimated by Dr. Thoroddson to have outpoured 

 twelve or thirteen cubic kilometers, or three cubic miles of lava. 

 The greatest eruption of which we have any estimate — and that is a 

 very crude one — was at Tomboro, on the island of Sumbawa, 

 which was estimated to have discharged about six cubic miles of lava. 

 This estimate is very excessive, and is probably two or three times too 

 large. 



On the same scale as before these two eruptions are represented, 

 and you perceive how insignificant they are in mass in comparison 

 with the whole of the surrounding earth. 



3. The third general fact is the repetitive nature of volcanic erup- 

 tions. A single outbreak, with none following, is an exceeding rare 

 phenomenon. Many eruptions, going often into the thousands, occur 

 before the climax is reached and the decline of activity follows. The 

 reason why a volcano, when its vent is once open, does not discharge 

 all the material in its reservoir in one stupendous belch, and then 

 close up forever, will be shortly brought up. 



4. The next general fact, which we cannot claim to be proven, but 



