882 PRINCIPLES OF STRATIGRAPHY 



The first type of seismic disturbance may be spoken of as bary- 

 seismic,-" and the second as pyroscismic.'\ these terms indicating 

 the relationship of the disturbances to the respective spheres. 

 Hoernes (19) has designated as a third type the results of incaving 

 of the roofs of fissures {Einsturzbchcn) which characterize the 

 Karst region of the Dalmatian coast. This, however, is to be classed 

 as a special phase of the dislocation (baryseismic) type, since such 

 cavings-in of cavern roofs are merely special phases of faulting. 

 In the same way, we must class under the volcanic or explosive 

 (pyroseismic) type the tremors resulting from explosions of gin- 

 powder or dynamite, and of gases, in mines and elsewhere, which 

 may not be sufficient to .affect the seismograph, but are certainly 

 noticeable as sea-quakes (submarine explosions) and as air-quakes. 

 These, as well as the disturbances due to incaving, may be dis- 

 missed without further notice. 



The Volcanic or Pyroseismic Type of Earthquake. This is, of 

 course, an accompaniment of volcanic activities ; but such dis- 

 turbances are not necessarily always felt, for, even if they occur, 

 they may be so slight as to escape notice. 



The Tectonic or Dislocation (Baryseismic) Earthquake. This 

 is a jar occasioned by the breaking of rock under strain. "The 

 strain may be caused by the rising of lava in a volcano or by the 

 forces that make mountain ranges and continents." The rupture 

 of the rock mass "may be a mere pulling apart of the rocks, so as 

 to make a crack, but examples of that simple type are compara- 

 tively rare. The great majority of ruptures include not only the 

 making of a crack but the relative movement or sliding of the rock 

 masses on the two sides of the crack; that is to say, instead of 

 a mere fracture, there is a geologic fault." (Gilbert-13 iP.) 



The walls of the fault plane may eventually become cemented 

 together, but they will remain as a plane of weakness for a long 

 time, so that repeated slipping may take place, making the region 

 one of frequent earthquakes. This has been the case in the repeated 

 California earthquakes, of which the San Francisco quake of 1906 

 is the most recent. The fault-line there extends for several hun- 

 dred miles northwest and southeast and nearly parallel to the 

 coast. The "Fossa Magna" crosses Japan from north to south, 

 while the southern border lands of Afghanistan have such an 

 habitual earthquake-producing fault-line extending for 120 miles. 



The faulting or slipping which produces the earthquake may 



*From the Greek IBap6s = heavy, -\- ffelcfia = earthquake; signifying that 

 weight or gravity is the dominant factor in their production, 

 t From the Greek nvp = fire, + aehiw. = earthquake. 



