The Causes and Phenomena of Earthquakes. 71 



the effects of gases, the results being proportional to the nearness 

 of the disturbing cause to the surface. The second class of 

 earthquakes he considers connected with the tumbling in of the 

 covering of immense caverns in the interior crust of the earth ; 

 the third class he assigns to electricity in the earth and atmos- 

 phere, seeking to establish an equilibrium. 



He gives a mathematical determination, illustrated by a dia- 

 gram, showing that earthquakes are to each other inversely as the 

 squares of distance from the point of disturbance. (Tome I, p. 

 388.) 



In summing up, he considers earthquakes to be precursors of 

 volcanic eruptions, and states certain conditions of the air and 

 sea, remarking on the extraordinary and unusual sensations of 

 men and other animals. Some of his conclusions are confirmed 

 or adopted by Mallet, and are also given by other writers. 



But Mallet, gives a further useful rule, by which we may judge 

 of the class of earthquakes. Thus the radius of a great one is 

 considered to be 9°, or 540 miles ; of the next class, 3°, or 180 

 miles ; and of the third, 1°, or 60 miles geographical. This 

 would again place the earthquake of 18th June in the second 

 class, or intermediate with Nos. 2 and 3. 



It cannot be supposed that the preceding discussion has ex- 

 hausted all the topics connected with the subject. Yet, it is 

 perhaps, almost too much extended for time to make the appli- 

 cation proposed to be made of it. It has, however, appeared to 

 me advisable to present the deductions of different authorities, 

 in order to make some explanation possible. 



I have only to regret that, in entering upon an attempt of the 

 kind, we have so few local data. Yet, in dealing with one part 

 of the subject, — viz., whether there is any change going on in 

 this part of the world from earthquakes, we must not neglect two 

 important considerations, viz., that the Pacific contains both a 

 subsiding and a rising area, as shown by the researches of Darwin 

 and Dana, and that (so far, at least, as we can discover) there is 

 evidence over large areas, in the present epoch, of one of 

 rest ; whilst on some parts of the shores of Eastern Australia 

 there are, notwithstanding, evidences of elevation, which is also 

 the case in New Zealand, as emphatically proved by the earth- 

 quakes of 1855. Our late shock was probably one of depression. 



"We must also bear in mind that, generally speaking, as shown 

 by Mallet, the result of all earthquakes is not the production of 

 permanent elevation, and that they generally indicate final sub- 

 sidence instead. But permanent elevation may accompany them, 

 produced by other and connected causes, as shown by Hopkins, 

 either the great elevatory forces within the earth, or secondary 

 operations belonging to them. 



An earthquake, then, may be held as the result of elevation, but 

 not as the cause. 



