284 HOBBS— THE EVOLUTION AND THE [April 24, 



into a mosaic of blocks, and that these Hnes of fracture may there- 

 fore be designated seismotcctonic or volcanotectonic lines or simply 

 lineaments according as they are revealed by earthquakes, by volcano 

 rows, or by topographic and geologic peculiarities. 



A Possible Explanation of " Volcanic Earthquakes." — Writing 

 before 1885 Suess distinguished two classes of earthquakes, the dis- 

 location and the volcanic earthquakes, and to these Rudolph Hoernes 

 added the type of in-caving earthquakes to cover especially some 

 of the light shocks of the Dalmatian coast. If we were to supply 

 a complete category of earthquakes it would be necessary to add 

 further a type of cataract earthquakes to cover the occasional fall 

 of limestone blocks in the Niagara cataract, as well as many other 

 minor forms, such as blast shocks in mines, etc. In point of im- 

 portance two classes only stand out sharply as they were originally 

 announced by Suess, and the present writer has been of the opinion 

 that even these may perhaps be subclasses only of a single phenome- 

 non. The mechanics of volcanic eruption, so far as it applies to 

 the cone, is now so well understood that we are able to connect 

 the outflow of lava which marks the beginning of the grand stage 

 of paroxysmal eruption in a composite cone, with the rending of 

 the mountain and the opening of a fissure — a distinctly tectonic 

 movement induced by the lava as it rises under the influence of 

 gravity, aided perhaps by the expansive power of the associated 

 steam. I believe we have been misled into supposing that the 

 fissures which are thus opened are necessarily radial to the cone, 

 since this would be presumed if the mass of the cone and its base- 

 ment were throughout homogenous, with no preexisting fractures, 

 and were acted upon by hydrostatic pressure from the central 

 shaft only. 



Etna is a giant mountain rising nearly 11,000 feet directly from 

 the sea, its diameter is more than twenty-five miles, and since the 

 higher portions are so largely concentrated at the center, the aver- 

 age thickness of visible volcanic ejectamenta over the base of the 

 cone is only about one half mile. Apparently, therefore, this super- 

 ficial layer of volcanic material may play a relatively small role in 

 the rending of the entire mass which accompanies an outflow of 

 lava. Sci soon as we examine the lines of parasitic craters which 



