VOLCANOS AND RADIOACTIVITY 545 



the epicentrum. It often happens that the intensity is so much affected 

 by the local nature of the soil and rocks that all estimates become so 

 uncertain as to be very misleading, and all attempts to draw conclusions 

 from them must be affected by large errors or may fail entirely. On 

 the other hand, in many cases the results are safer and surer than would 

 be supposed, and we are able to give a graphic representation of the 

 curve of intensity which must be very near the truth. In general, 

 when an earthquake is very strong at the epicenter and quickly fades 

 out away from it, we can say with confidence that its centrum is very 

 shallow. If the intensity fades out slowly and the quake is felt at 

 great distances, we can rely upon its centrum being very deep. When, 

 therefore, we have not the means of estimating the intensity at the 

 critical points, if we have the means of estimating the maximum in- 

 tensity of the quake and of knowing how far it is felt, we can still 

 form, not, indeed, a precise or accurate estimate of its depth, but a 

 roughly approximate one. 



A qualification of the foregoing may be introduced here. The 

 earthquake is no doubt the fracturing or sudden yielding of the rock 

 masses immediately above the lava reservoir. We can only vaguely 

 conjecture the distance which separates the zone of fracture from the 

 zone of melting. But in no case could it be so great as a mile without 

 making itself sensible in the greater depth of the quake. We must, 

 however, increase slightly our estimate of the depth of the lava beyond 

 the estimated depth of the quake. 



We may now proceed to state the probable cause of volcanic erup- 

 tions. They are caused, I conceive, by a development of heat resulting 

 from radioactivity in limited tracts at a depth of one to three — at the 

 very utmost not over four — miles from the surface, which is sometimes 

 sufficient to melt the rocks affected by it. The melting is gradual, and 

 when a sufficient quantity is melted, the water which it contains be- 

 comes explosive and usually suffices to break through the covering, 

 constituting an eruption. When all the lava is erupted, and the reser- 

 voir is exhausted, it closes up for a time. If the heat continues to be 

 generated, more lava is melted, and in due time another eruption occurs. 

 The process may be repeated again. It may be repeated hundreds or 

 thousands of times. The volcanic action may continue in the same 

 place for hundreds of thousands, or even millions, of years, or it may 

 repeat itself only a few times, or may even occur only once. Indeed, 

 it may fail altogether to erupt to the surface, and in many cases does 

 fail. In other words, it goes through the entire process of preparing 

 for an eruption and does not consummate it. 



This view enables us to explain the repetitive character of volcanic 

 eruptions, which is, perhaps, their most striking and characteristic 

 feature. It is in strong contrast with the view long held that the lava 



vol. iixvni. — 35. 



