16 On the Recent Earth-Tremors, 



emanated from a point situated low down in the outer 

 crust. The greatest depth at which such movements can 

 occur is believed to be 30 miles, and most earthquakes 

 originate at depths of from 8 to 15 miles. I am of opinion 

 that, with ours, the latter distance must have been the 

 minimum depth. The exact depth of the focus can be 

 calculated by observing the mean direction of the rents in 

 the walls of buildings, and I regret that I have no data of 

 this kind to offer. 



The next feature in connection with this earthquake which 

 I shall point out is that the seismic activity has returned 

 with the winter season. The previous shock occurred in 

 June of last year. The records of the northern hemisphere 

 show that three-fifths of the whole number of earthquakes 

 there occur in the winter season, and it is interesting to see 

 that the connection with that season is maintained in this 

 hemisphere. 



Another interesting circumstance is that the focus occurs 

 within an area of high barometric pressure, as is well shown 

 by the weather chart of that date. The isobar of 302 

 surrounds it, and that of 30" 3 overlies Bass's Strait. Vol- 

 canic disturbances increase with a low barometer ; with us 

 the converse has occurred, and this fact is important when 

 we are investigating the causes of the recent tremors. 



II. 



Having described the features of this earthquake, I will 

 proceed to consider what are the subterranean conditions 

 which they indicate. An earthquake is the result of an 

 earth-rupture, and an earth-rupture is caused in three 

 different ways. It may occur through the contraction of the 

 globe, due to loss of heat, during which secular process 

 the inner cooling parts shrink away from the cold solidified 

 external shell, which latter part nevertheless clings to the 

 retreating nucleus under the indraught of gravitation. This 

 crust, ever contracting, and yet ever too large, unceasingly 

 adjusts itself to the dwindling core by buckling up along 

 certain lines, and then cracking along the folds or anticlinal 

 crests. The ruptured edges, pressed together, stack up their 

 shattered strata into mountain chains. In other words, 

 mountains are the result of the crumpling and piling-up of 

 broken strata under the pressure of a tremendous tangential 



