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-Brine/ie/d Subsidence at W~ndso ; Oetar~ the hiiddlc Salts sagged into one or more under- lying cavities. t\lore or Icss simultaneously, it may be supposed, similar sagging took place into the cavity or cavities in the bliddlc Salt. The strata oC the Bass Island dolomite overlying the Upper Salt ~verc initially thick enough to span a cavity in the Upper Salt without appreciable de- formation. Eventually, however, partings along bedding planes must have opened, whereupon one or more strata descended into the cavity, possibly without disruption or notable bulking. As succes- sive strata were detached from the roof and came to lie on the floor of the cavity, the load on the salt remnants at various deeper levels was increased and mast have produced failure of some of the salt pinnacles which provided support for the overlying broken strata. Concurrently, solution w•as taking place in the salt beds, owing to the fact that water was being introduced through well No. 19, west of the subsiding area, while brine was being pumped from wells located east of i[. Both the increasing load on salt remnants and their continued diminu- tion by solution must have contributed to a gradual increase in the settlement of the overlying broken strata resting on them, and hence to an increase in the height o.f the cavity spanned by intact rock. There seems to be no way of determin- ing the stage at which the vertical dimension of this cavity became so great that there vas notable shat- tering and bulking of the strata as they dropped into it. It is only known that when the roof finally failed, [he vertical dimension of the cavity ex- ceeded that of the broken rock by twenty-five feet 307 in the deepest p•, and b}' only a few inches at a distance of several hundred feet from the center. The remainder of the cavity was completely Cillcd with broken rock, either at the time of sinkhole Formation or in the few months prior thereto. Con- sequently little or no sudden subsidence took place in the area located more than some two hundred Feet beyond the edge of the sinkhole area. REFERENCES Bays, Carl A., 1954, Subsurface conditions at the Plants of Canadian Industries, L[d., and Cana- dian Salt Co. with reference to the subsidence oC February 19, 1954: unpublished report. Cale}', J.F., 1945, Paleozoic geology of the Windsor-Sarma Area, Ontario: Canadian Geolog- ical Survey Memoir, No. 240. Landes, Ii.I~., 1945, The Salina and Bass Island rocks in the Michigan Basitt: 1•lichigan Geologi- cal Survey, Div. Oil and Cas Investigations, Prelim Alap 40. Peck, Ralph B., 1954, 1`lemorandum concerning subsidence at Windsor, Ontario: unpublished re- port. Terzaghi, liarl, 1950, i\lemorandum concerning the subsidence of brine fields: unpublished report of Dec. 30. ---, 1954, Report on the subsidence of Febntan' 19, 1954, in Windsor, Canada: unpublished re- port of Oct. 27.