284 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



earthquakes according to their field of action as succussive or 

 vertical, undulatory or wave-like, and rotatory or whirled. 

 At the present day, earthquakes are usually classified as 

 central and linear; in the case of "central" earthquakes the 

 undulatory movements radiate from a seismic focus towards 

 all directions; in the case of "linear" earthquakes, the 

 movements are limited to long strips of the crust. Von 

 Seebach termed the subterranean origin of an earthquake the 

 " seismic centre " ; the median point at the surface within a 

 region of earthquake shock he termed the "epicentrum" at 

 this point the shock manifesting itself chiefly by up and down 

 motion ; and to the imaginary lines drawn through all points 

 simultaneously affected by the shock, he gave the name of 

 "homoseisms" or "isoseisms." But it has to be remembered 

 that a definite central point of origin has only been determined 

 in a few cases. Generally the seismic centre or focus has 

 been ascertained to be in point of fact an underground area 

 from which concussions are propagated vertically along a 

 large number of parallel lines, which Mallet has called 

 " Seismic Verticals." Undulatory impulses are also transmitted 

 obliquely through the surface, the intensity of the shock at the 

 surface diminishing in proportion as the angle of emergence 

 increases. In the case of the Agram earthquake in 1880, a 

 large surface area was affected by vertical movements of 

 almost equal intensity, showing that the underground focal 

 area was of considerable extent. 



The leading geological authorities now associate earthquake 

 shocks with manifestations of volcanism, crust collapse, or 

 tectonic crust-movement. Earthquakes as a rule precede or 

 accompany the eruptions of active volcanoes, but they often 

 occur in volcanic districts when there is no actual discharge 

 from volcanic vents. The earthquakes which have been 

 directly traced to crust subsidences were of small extent and 

 intensity. And it is now widely accepted that most earth- 

 quakes which occur in non-volcanic districts are originated by 

 dislocations and movements in the earth's crust. 



In two suggestive papers (1873-74) on the Earthquakes of 

 Lower Austria and Southern Italy, Professor Suess showed 

 conclusively that earthquakes occur along the lines of tec- 

 tonic movement in a mountain-system, and quite irrespective 

 of any volcanic phenomena. Hoernes contributed several 

 interesting papers on tectonic tremors, demonstrating by 



