Crosby.] 78 [October 4, 



tical fissures. There can be no doubt, however, that the primary 

 or causative fissures often approach much nearer the surface than 

 that in which the Neapolitan earthquake so carefully studied by 

 Mallett originated ; and it seems in the highest degree probable 

 that the New Zealand earthquakes of 1848 and 1855 are a common 

 type. These disastrous shocks were due to the sudden formation 

 of gigantic fissures which reached the surface, being traced as 

 open cracks — the first (in the south island) for a distance of 

 sixty miles south-west from Cook's Strait, and the second (in the 

 north island) for the almost incredible distance of ninety miles 

 north-east of the same channel. The second fissure was accom- 

 panied throughout its entire length by a permanent dislocation of 

 the rocks, the country on the north-west side being elevated nine 

 feet, while the south-west side remained stationary. 



Joints are, as a rule, most conspicuously vertical in horizontal 

 strata. But in such formations, especially if there are partings 

 or planes of weakness between the beds, the earthquake move- 

 ment need not be horizontal in order to produce vertical fractures, 

 for these are almost inevitable, even though the vibrations move 

 obliquely upward at a considerable angle, on account of the nat- 

 ural tendency of the beds to break at right angles. 



Earthquake fractures are necessarily parallel, and, although 

 portions of curves, yet for the most part of curves so great as to 

 appear sensibly straight in any single exposure, or even in a series 

 of exposures extending many miles. Here again they are essen- 

 cially identical with joints. Joints are not only parallel, but in 

 the same district, the principal sets usually show considerable 

 constancy in direction. Now earthquake foci, so geologists gen- 

 erally belie ve, are situated along certain definite lines in the 

 earth's crust — sometimes lines of volcanic activity, but more 

 commonly lines of upheaval, plication and faulting. In other 

 words, earthquakes usually originate in the axes of mountain- 

 ranges, or in zones parallel with these. And this is simply equiv- 

 alent to saying that, as a rule, we may expect to observe consid- 

 erable constancy in the directions of earthquake-movement and of 

 earthquake-fractures in districts of limited extent. 



It is a familiar fact that the faults, veins and dikes of a district 

 are usually parallel with the principal sets of joints. But this 

 relation is easily and naturally explained, if we accept earth- 



