sea caused by the alternate thawing and refreezing of the northern icecap. When each sequence of the ice age ended, the land bridge remained, high and dry as ever, and it was over this isthmus that paleolithic man shambled across from the Continent, in the trail of rhinoceroses, hippopotamuses, giant boars, and other great beasts whose fossilized bones have been found in the Wealden area. Encroaching seas made a channel through the isthmus and cut the Bronze Age descendants of this breed of men off from the Continent about six thousand years ago. Then fierce tidal currents coursing between the North Sea and the Atlantic widened the breach still further until, as recently as four thousand years ago (or only about a couple of thousand years before Caesar's legions invaded Britain by boat), the sea wore away the rock of the isthmus to approximately the present width of the Strait, leaving exposed high at each side the eroded rock walls, formerly the whitish mudbank of Cretaceous times—now the white chalk cliffs of the Dover and Calais areas. Providentially for the later purposes of Channel tunnelers, however, the seas that divided England from the Continent also left behind them a thin remnant of the old land connection in the form of certain chalk layers that still stretched in gentle folds across the bottom of the Strait, and it was through this area of remaining chalk that the Victorian engineers planned to drive their tunnel headings. Even more providentially, they had the opportunity of extending their headings under the Channel through a substratum of chalk almost ideal for tunneling purposes, known as the Lower Chalk. Unlike the two layers of cretaceous rock that lie above it—the white Upper Chalk and the whitish Middle Chalk, both of which are flint-laden, heavily fissured, and water-bearing, and consequently almost impossible to tunnel in for any distance—the Lower Chalk (it is grayish in color) is virtually flint-free and nearly impermeable to water, especially in the lower parts of the stratum, where it is mixed with clay; at the same time it is stable, generally free of fissures, and easy to work. From the coastline between Folkestone and South Foreland, north-east of Dover, where its upper level is visible in the cliffs, the Lower Chalk dips gently down into the Strait in a north-easterly direction and disappears under an outcropping Middle Chalk, and emerges again on the French side between Calais and Cap Blanc-Nez. Given this knowledge and their knowledge of the state of Lower Chalk beds on land areas, the Victorian engineers were confident that the ribbon of Lower Chalk extending under the Strait would turn out to be a continuous one. To put this view to a further test, the French Channel Tunnel Company, in 1875, commissioned a team of eminent geologists and hydrographers to make a more detailed survey of the area than had yet been attempted. In 1875 and 1876 the surveyors made