There is a regrettable tendency to looseness of thought among geologists on the subject of isostasy. I give no quotations in support of this assertion, firstly because, being always in the form of casual allusions to the principle, they would require long hunting down; and secondly because, if given, they would fasten upon a few individual geologists a criticism which should be more general. The usual form in which the looseness of thought shows itself is in explanations of shallow-water deposits of thickness greater than their depth of accumulation. We are frequently told that such thick deposits result from local subsidence due to the loading of the sea-floor by the great weight of sediment, and reference is made to the principle of isostasy as justifying this explanation.