SECTIONAL TRANSACTIONS— A. 391 



or three cycles of loading and unloading to remove a set on no load. Further 

 cycles resulted in complete recovery, with a hysteresis loop. Very pro- 

 nounced increase in deformation took place when the load on the rock beams 

 was maintained constant, and in some rocks these time strains increased 

 with an increase of load up to a certain load, the time strain for higher loads 

 progressively decreasing. 



In a few cases the further deformation with time had been allowed to 

 progress until finally the beams fractured, some under loads which were 

 less than the loads, applied without allowing time effect, necessary to 

 fracture similar beams cut from the same rock samples. 



When subjected to torsion these rocks exhibited a time effect as in the 

 case of compression and bending. 



Mr. T. F. Gaskell. — Seismic exploration of eastern England (12.0). 



The surface rocks of East England consist of Jurassic and Cretaceous 

 clays and chalk. It has long been known that these are underlain by a 

 planed-oflF surface of Palaeozoic rocks, but the depth of this surface was only 

 known at a few isolated points. The refraction methqd of seismic pro- 

 specting has been applied to map this surface. Charges of gelignite up to 

 15 lb. are used to make impulses in the ground, and six electrical seismo- 

 graphs record the arrival of the waves produced. The recording apparatus 

 is transported in a van, and depths of the Palaeozoic have been determined 

 at stations distributed over a large area of Eastern England. 



Afternoon. 

 Symposium on Seismology (continued) (2.20). 



Dr. H. Jeffreys, F.R.S. — Deep foci and aftershocks. 



The work done by Dr. Phillips throws light on some difficult seismological 

 questions. Two hypotheses concerning the mechanical properties of rocks 

 are that of a finite viscosity at small stresses, or zero strength, and that of 

 an infinite viscosity at small stresses, or finite strength ; the former is 

 associated most prominently with the name of Wegener, the latter with 

 that of Barrell. The latter agrees better with the distribution of gravity 

 anomalies, and with the fact that stresses capable of producing major earth- 

 quakes can develop at depths down to about 700 km. It has not, however, 

 been easy to see how the idea of perfect elasticity up to a definite limiting 

 stress can be reconciled with the existence of aftershocks continuing for 

 months after the main shock. Dr. Phillips's work shows how this can be 

 done, by the recognition of the distinction between the stress that leads to 

 immediate fracture and that which leads to fracture only if it is left on long 

 enough. 



The intensive study of deep focus earthquakes is likely to lead to solutions 

 of some difficult seismological problems that are almost insoluble from the 

 data of normal shocks, namely the depth of the core, the nature of the 

 20° discontinuity, and the times of the transverse wave up to distances of 

 about 25°. 



Dr. R. Stoneley, F.R.S. — Times of travel of the L phase (2.50). 



It was found by Prof. H. H. Turner that L readings given by stations 

 have travel times equivalent to 0-48 min. /degree, but that sometimes onsets 

 corresponding to 0-41 min./degree predommate. The former correspond 



