VOLCANOS AND RADIOACTIVITY 261 



for which there is a growing mass of strong and highly concordant evi- 

 dence, is that the seat of the reservoir is very shallow and seldom more 

 than three miles deep. Very rarely is there any indication of its being 

 more than two and one-half miles deep, and it is certain that in many 

 cases the depth is less than one mile. The indications are that most 

 of the volcanic eruptions originate at depths between one mile and two 

 and one-half miles. The evidence of this is furnished by the earth- 

 quakes which almost always accompany them, and which are associ- 

 ated with them in such a way as to leave no doubt or question that 

 they are produced by the volcanic action. The radiation of the 

 tremors of an earthquake from their source in the earth is governed 

 by substantially the same law as sound. The intensity of these 

 tremors, where they reach the earth's surface, varies in a manner 

 which is dependent upon their depth of origin. In the discussion of 

 the Charleston earthquake I pointed out one method by which that 

 depth can be approximately computed from the distribution of critical 

 points of the surface intensity. The method has been sharply criti- 

 cized by able seismologists as being hable to error through refraction 

 of the rays of propagation through rocks and media of variable 

 density. But I observe that all of them use that method with 

 surprising consistency and satisfactory results. 



The efficiency of this method depends mainly upon the accuracy 

 with which the intensity can be estimated along a hne radiating from 

 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 conclu- 

 sions 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 inten- 

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



