HuTTON. — The Earthquake in the Amuri. 291 



he felt a series of strong upward shakes in the ground. I 

 think therefore that at AVoodbank the shock must have been 

 locally increased by reflection from below ; but it is not easy 

 to say why this should have taken place. The house stands 

 at the northern margin of the alluvial plain, but a spur of the 

 hills comes down between the house and the origin of the 

 earthquake so far that the end of the spur bears south-west from 

 the house. This spur ought to have prevented total reflec- 

 tion upwards from taking place. The only suppositions that 

 occur to me are that either the shocks here came from more 

 to the south — that is, from the valley of the Waiau-ua — or 

 else that an undergi'ound spur exists from the hills east of 

 "Woodbank, w^hich would act as a reflector of the earth-wave. 

 Neither of these suppositions appears to me to be probable. 



At the Hanmer Hot Springs the intensity of the shock was 

 probably that due to the distance (31i miles) from the 

 epicentrum, neither augmented nor diminished. 



Cause of the Earthquake. 



All ordinary earthquakes are due to one of two causes — 

 they are the result either of subterranean explosions of steam 

 or of the crushing or fracturing of rocks. There may occa- 

 sionally be an earthquake due to some other and exceptional 

 cause, but these must be rare. 



The first cause — explosion of steam — is due to water 

 coming into contact with heated rock ; and, as the water must 

 originally have been sm'face-water — i.e., rain-water — the 

 heated rocks must be near the surface of the earth, and the 

 centrum of the earthquake must also be more or less super- 

 ficial. These earthquakes are usually found associated with 

 volcanoes or in hot-spring districts, and always occur previous 

 to and during the progress of an eruption ; although they also 

 often take place without an eruption. The earthquakes felc 

 in the hot-spring district from Lake Taupo to Kotorua are no 

 doubt of this origin. 



The second cause — fracture of rocks — is due to the gradual 

 increase of strain on the rocks until at last they give way with 

 a more or less sudden snap and jar. These strains may be 

 due to lateral pressure in the earth's crust, which crumples it 

 up and forms mountain-chains ; or they may be due to the 

 gradual relief from pressure owing to the denudation of the 

 surface, which is more rapid in mountain-ranges than else- 

 where ; or they may be due to the gradual increase of pressure 

 brought about by the deposition of mud or sand on the sea- 

 bottom, a cause which acts most rapidly near the mouths of 

 great rivers. Earthquakes due to this cause may occur in 

 almost any part of the world, but they are most numerous in 

 mountain-ranges and near the mouths of large rivers. Many 

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