380 SEE -THE CAUSE OF EARTHQUAKES. [October 19, 



it forces its way into a fissure to form a dyke which may never reach the 

 surface. In one place a long arm or sheet of lava has in a most surprising 

 and inexplicable manner thrust itself into the enveloping rock-mass, and in 

 the older of metamorphic rocks these offshoots or apophyses cross each other 

 in great numbers and form a tangled network of intrusive dykes. In other 

 places the intruded lava formed immense lenticular masses (laccolites), which 

 have domed up the overlying strata into mountain masses. These intrusions, 

 almost infinitely varied in form and condition, are often, in fact usually, in- 

 explicable as mechanical problems, but their reality is vouched for by the 

 evidence of our senses. What concerns us here is the great energy which 

 they suggest and their adequacy to generate in the rocks those sudden, elastic 

 displacements which are the real initiatory impulses of an earthquake. They 

 assure us that a great deal of volcanic action has transpired in past ages far 

 underground, which makes no other sign at the surface than those vibrations 

 which we call an earthquake." 



The blowing out of a huge obeHsk of granite some 300 meters 

 long and 120 meters in diameter but too large to get through 

 the orifice of Mount Pelee, and which therefore hung in the mouth of 

 that volcano, while the cone itself was split on all sides by the fearful 

 force of the explosion which had ejected the plug of granite from 

 the roots of the volcano, affords an excellent and familiar example 

 of what may be done by volcanic forces. This obelisk is illustrated 

 in the National Geographic Magazine for August, 1906, by photo- 

 graphs made by Professor A. Heilprin, who gives an excellent ac- 

 count of the history of the obelisk and its gradual disintegration. 

 If these forces are suddenly arrested, as they are in all earthquakes 

 which do not produce immediate eruptions, the shock is taken up by 

 the surrounding earth and we have a violent earthquake which may 

 be felt all over the world. 



If the explosions are no larger than those involved in the erup- 

 tions of geysers the results are mere microseisms or earth tremors, 

 interesting enough to be sure, but of an unimportant character. A 

 really serious earthquake, to be felt all over the world, implies the 

 exertion of the most tremendous forces, and the way in which these 

 forces set the earth particles vibrating shows that they must depend 

 primarily upon the explosive power of steam-saturated lava at great 

 temperature. The result is a violent shaking of the whole overlying 

 layers of rock and the occasional upheaval of volcanoes where the 

 strata are fractured and weak. The very way in which the earth 

 twists, heaves, labors and vibrates shows the awfulness of the pent-up 



