The forces which produced the folds of the Alps, and caused waves of compression to be transmitted as far as the Weald, may have been sufficiently powerful to start a drift displacement of Europe towards the north-east. As there does not appear to have been any similar compression in North America, a north-easterly movement of Europe would have caused shear in the strong continental layers which are considered to have connected the two continents. Tensile stresses would probably have arisen in these layers, unless the layers had begun to thin out before the state of tension was reached. There would have been a limit to the extent to which thinning could have proceeded, since a certain thickness of rock would have been needed to produce the necessary stress-differences. This thinning out process may have come into operation, except in a number of kilometres immediately below the surface, where tensile stresses may have arisen as drift continued. Under these circumstances the uppermost part of the crust would have sagged upon the thinning region, nd been broken up by a system of tension fractures.