Hogben. — Theory of New Zealand Earthquakes. 505 



west.* This tilting took place suddenly ; but it is interesting 

 to note that for some months before the earthquake a gradual 

 lowering of the level on the west side had been taking place, 

 and that a similar gradual movement has been going on up 

 to the present time (September, 1905). The 1901 earthquake 

 therefore gives us an example of tilting, or folding, in which the 

 original fracture of the rocks did not apparently extend to the 

 surface. 



It is well known that strata originally more or less horizontal 

 have been folded by forces acting on the earth's crust, the folds 

 often appearing at the surface so as to form wrinkles or mountain 

 ranges and valleys. Several possible causes of this folding may 

 be assigned. Two of the most important are — (1) the gradual 

 cooling of the earth ; (2) the loading of the ocean-bed through 

 denudation of the land-surface. 



(1.) As to the cooling of the earth : The temperature of the 

 rocks near the surface, being the temperature of space modified 

 by the heat from the sun's rays, is practically constant ; so 

 that those rocks will not contract. All other layers of the crust 

 will cool and contract at rates varying with the depth. The 

 volume of the layers in which cooling is greatest will, after con- 

 traction, be too small to fill the space into which the layers fall, 

 and they will therefore be pressed out by the weight of the rocks 

 above them. But the upper strata, which cool less quickly, will 

 be too large to fill the space into which they fall, and their sur- 

 face will accordingly be crumpled. The effect will be to pro- 

 duce a series of elevations and troughs (anticlinal and synclinal 

 folds). The folding or crumpling is determined partly by pre- 

 viously existing lines of weakness, partly by inequality of ver- 

 tical pressures due to differences of texture and density in the 

 upper layers. Unequal vertical pressures on adjoining portions 

 of the earth's crust will cause unequal lateral pressures, and a 

 tendency for the rocks to " creep " or move horizontally in order 

 to repack themselves in a more stable condition. This lateral 

 thrust, again, may produce fractures, reversed faults, and eleva- 

 tion of those strata which are less dense or subject to a smaller 

 vertical pressure. Obviously, earthquakes will occur whenever 

 any of these movements are sudden in character. 



(2.) " Loading " : Another cause of folding is the trans- 

 ference of material from the land-surface to the bottom of the 

 sea by the agency of rivers : the pressure on the strata un- 

 derlying the land will be thereby relieved, and the pressure on 

 the floor of the ocean will be increased. There will thus be a 



* In the 1855 earthquake the angle of tilt could not have been less 

 than 4 seconds, and may have been as much as 10 seconds. 



