Current theories on geosynclinal subsidence and on the disappearance of ancient land masses postulate crustal thinning by subcrustal processes. This idea was a necessary implication of the two premises popular a decade ago, namely: (1) surface elevation is related to crustal thickness according to Airy's model of isostatic compensation; (2) either the crust as defined by the Mohorovicic discontinuity is comparable with the crust envisioned by Airy, who separated a solid crust from a fluid substratum, or there is no significant regional difference in mantle density, and there has been none during the past. Recent geophysical investigations have indicated that isostatic adjustment could result not only from changes in crustal thickness, but also from variation in upper mant e density. This new discovery permits the postulate that thinning of continental crust can be related to removal of surficial sialic material by supracrustal processes, such as erosion, gravity sliding, and overthrusting from elevated regions which owed their surface elevation to an abnormally low mantle density. Subsequent increase in mantle density because of variations in mantle temperature leads to isostatic subsidence of regions of thin crust. Such a combination of crustal thinning by supracrustal processes and isostatic subsidence related to mantle density changes could explain adequately (1) the disappearance of ancient land, (2) the formation of a new geosyncline at the site of an ancient land, and (3) the estimated chemical composition of the earth's crust. The efficacy of supra rustal processes to remove large quantities of sialic materials has been discussed. It is not necessary to postulate subcrustal processes which either must assume lateral transfer of vast quantities of crustal material or that the Mohorovicic discontinuity is a phase change boundary. An isostatic model relating surface elevation, crustal thickness, and mantle density variation during an orogenic cycle is presented.

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