WILLIAM G. STEVENSON. 205 
nevertheless, it seems to me that geology has furnished 
abundant evidence, in the fissures and faults of the 
surface rocks, to justify an inference as to the nature 
and origin of the mighty blow which sends an impulse 
over a continent, and makes the earth tremble as 
though it were a reed shaken by a2 storm. 
In an interesting discussion which followed this ad- 
dress Prof. W. B. Dwight reviewed the many and various 
phenomena which must all be explained by any ade- 
quate theory of the origin and nature of earthquake 
motion. He gave reasons for believing that though 
many of the less important local earthquakes are doubt- 
less due to volcanic action, the chief cause of important 
and extended earthquakes is a displacement of the 
Strata by fracture due to contraction of the crust of the 
earth. As a condition to such contraction from cooling, 
there may be a plastic zone of molten rock at some dis- 
tance below the earth’s surface. Sir William Thomson 
has withdrawn some of the extreme conclusions which 
he had arrived at in opposing the theory of the possi- 
bility of a molton zone; and the arguments based on 
mathematical calculations which he still uses against 
this theory are regarded by many eminent geologists as 
having a fatal weakness because they are based on con- 
ditions supposed to exist beneath the earth’s crust, but 
yet entirely unknown. The presence of a molten zone 
can neither be proved nor disproved until these condi- 
tions are better Known. It is certain, however, that Sir 
William Thomson’s conclusion as to the rigidity of the 
earth’s crust is not in accordance with the facts ; for the 
fact is that folding, fracture, displacement, and yielding 
in all directions aré ubiquitous and constant phenomena 
in the history of the rocks. 
- The theory that earthquakes are due mainly to dis- 
placement resulting from contraction due to cooling is, 
so 
