In regions which are seismically active at the present time, stress differences are presumably increasing to the point where the shearing strength of the rock is locally exceeded leading to dislocation accompanied by earth- quakes. In addition to the localised areas where rock failure occurs, it appears reasonable to suppose that there is a much more extensive surrounding region influenced by the same tectonic forces but to a degree insufficient to cause failure of the rocks.

Seismic activity on a minor scale may be far more general then is generally recognized, since many seismograph installations are designed primarily to detect the effects of large distant earthquakes, and are relatively in- sensitive to small local shocks. A network of seismographs recently installed in the Snowy Mountains, especially designed to detect local seismicity, has recorded a considerable number of very small tremors of local origin, most of which would not have been detected by seismographs outside the area.

CONCLUSION

It is apparent from the determinations already made that the horizontal stresses in rock at depth cannot reliably be predicted. Because of their important effect on the behaviour of underground openings, it is very desirable that actual measurements should be made, where important underground operations are being undertaken.

ACKNOWLEDGMENT

This paper is presented by kind permission of the Snowy Mountains Hydro-electric Authority. The views expressed here in are those of the author and not necessarily of the Authority.

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