J. T. Jutson — Interpretation of Dry Lakes. 305 



a fact that might have been somewhat surprising if we had not been 

 partially prepared for it. The Chalk is about 150 feet thicker at 

 Beccles than at Lowestoft. The present writer has endeavoured to 

 show elsewhere that this area has been affected by posthumous 

 movements of an ancient axis which he considers to run parallel 

 with Kendall's Charnian Axis, eastward of Kent, into North France. 

 We notice here what we are also apparently able to detect in the 

 London area (see section), that during the deposition of the Chalk 

 (and possibly to some extent during the deposition of the Gault) 

 there was a definite movement of depression along the line of the old 

 axis whereby a somewhat greater thickness of sediment accumulated 

 vertically above them than elsewhere, Lowestoft lies somewhat to 

 the east of the line of the easterly axis and hence the decrease in 

 thickness of the Chalk is accounted for. On this view it follows that 

 a great thickness of Chalk must have been removed by denudation 

 from tlie district to the north-west of Norwich, since over 1,350 feet 

 of the formation yet remains beneath the Eocene in the neighbour- 

 hood of Mundesley and Happisburgh. 



With regard to the zoTial outcrops of the Chalk beneath the 

 Eocene cover in Suffolk and Essex, the Ostrea lunata zone evidently 

 does not occur here, but the line of separation of the Belemnitella 

 mucronata and Actmocamax qvadratus zones probably runs from 

 south-east to north-west beneath the Suffolk Eocenes, meeting the 

 coast somewhere in the neighbourhood of Harwich. After emerging 

 from beneath the Eocenes westward of Bramford the boundary turns 

 sharply to the north as the result of later denudation. The boundary 

 between the A. quadratics and Marsupites zones occurs a little to the 

 west of Hadleigh, and, beneath the Eocenes, probably sweeps off 

 to the south-west. The same may be said of the boundary between 

 the Marsu2)ites and 31. corangumum zones, which occurs a little to 

 the west of Sudbury. 



In conclusion, it may be remarked that when the Chalk of Essex 

 comes to be better known it will probably be found to be affected b}- 

 a great fault stretching across the whole county fiom the neighbour- 

 hood of Cliffe, in Kent, to near Dunmow and Thaxted, as a result 

 of which Chalk of the Marsupites zone, on the eastern side, is placed 

 in juxtaposition with the M. coranffuinum zone on the western side. 



The section accompanying the present paper is drawn from the 

 data supplied by maps showing contours on the Palaeozoic floor, in 

 the base of the Gault, and the base of the Chalk, prepared by the 

 Avriter at various times. 



IV. — On the Occurrence and Interpretation of Bock-Cliffs and 

 Bock-Elooks on the Western Shores of the "Dry" Lakes in 

 South- Central Western Australia. 



By J. T. Jutson, Geological Survey of Western Australia, Perth. 

 Introduction. 



IN South-Central Western Australia, in the physiographic division 

 which the writer ^ has termed the Central or Salt Lake Division, 

 in a large portion of which the average annual rainfall is about 



' "An Outline of the Physiographical Geology (Physiography) of Western 

 Australia " : Bull. 61, Geol.'Surv. W. Australia, Perth, 1914, p. 52. 

 DECADE VI. — VOL. V. — NO. VII. 20 



