The principle of isostasy means two things. First, it implies that an earth-shell beginning at a moderate depth from the surface of our planet is so weak that the material of the shell is nearly or quite in the hydrostatic state. For this weak shell Barrell coined the name “asthenosphere”. The word says literally that the shell has no strength whatever, but Barrell did not believe that it has no strength and invented the name merely to indicate relative weakness. Secondly, the principle declares that high segments of the overlying shell—the lithosphere—are in large part hydrostatically balanced by the low segments, the rugged relief of the globe being in other parts supported by the strength of the lithosphere.
It is also important to observe that isostasy does not necessarily imply a hydrostatic condition for any shell below the asthenosphere. The truth of the principle would not be . . .