In shape, the Great Landes may be compared to an immense rectangular triangle, having for its base the coast, which, from the mouth of the Gironde to Bayonne, or for a length of more than sixty leagues, is almost rectilineal. But they are separated from the sea by a long parallel chain of lakes and water-courses--a waste of shallow pools--a labyrinth of gulfs and morasses, and then by the continuous chain of the Dunes, of which we shall speak in the following chapter. That which is commonly called the Great Lande is bounded on the north by the _étang_, or lake, of Cazau. It is a sandy, treeless plain, and upon which, for a traject of several leagues from east to west, not one habitation worthy of the name is perceptible until the traveller arrives at Mimizan, near the southern point of the lake of Aureilhan. This lake on the south-west pours its waters into the sea. To the north it communicates, through the canal of St. Eulalie, with the lake of Biscarosse, which is itself connected with that of Cazau. East of this chain of lakes lies the Lande; west of it stretches the range of _Dunes_, or sand-hills. The lake or pool of Cazau is a small sea of fresh water, perfectly clear, profoundly deep, and fourteen to fifteen thousand acres in extent. It has its whirlwinds and its tempests, so that in certain seasons it is perilous to embark on its surface. And were its banks clothed with rich woods, or raised aloft in irregular or precipitous cliffs, it would surely attract as great a throng of tourists as the mountain-tarns and lochs of Scotland or Cumberland, or the Arcadian waters of Northern Italy. The lake of Biscarosse, in form a triangle, with one side formed by the Dunes, covers about twelve thousand acres. It derives its name from a village situated at its northern angle, on the bank of the canal which connects it with the lake of Cazau. The lake of Aureilhan is the smallest of the three; the St. Eulalie canal, which links it to the preceding, traverses a series of peat-bogs bounded eastward by gloomy pine-forests, and westward by the interminable Dunes, which, by arresting the flow of the rain-waters, have really created these so-called lakes and extensive swamps. Enormous quantities of rain fall every year in the Landes,--which district the Romans would certainly have dedicated to Jupiter Pluvius,--and find beneath the thin superficial stratum or crust of sand and earth, a sub-soil of _tufa_ and _allios_--in other words, of compact chalk and sand agglutinated by a ferruginous sediment. Frequently this _tufa_ possesses all the hardness of stone, and its imperviousness is its fundamental property. Hence it follows, that a portion of the heavy annual rainfall remains in the receptacles provided by the hollows and depressions of the soil, and in due time accumulates into marshes and lagoons, until gradually evaporated by the heat of spring.