These different elements in the region W. of the Mississippi are arranged from N. to S. in the order of decreasing geologic age and maturity. Beginning with elevations of about 400 ft. near the Arkansas line, there is a gentle slope toward the S.E. The northern part can best be regarded as a low plateau (once marine sediments) sloping southward, traversed by the large diluvial valleys of the Mississippi, Red and Ouachita rivers, and recut by smaller tributaries into smaller plateaus and rather uniform flat-topped hills. The "bluffs" (remnants of an eroded plain formed of alluvion deposits over an old, mature and drowned topography) run through the second tier of parishes W. of the Mississippi above the Red river. Below this river prairie areas become increasingly common, constituting the entire S.W. corner of the state. They are usually only 20 to 30 ft. above the sea in this district, never above 70, and are generally treeless except for marginal timber along the sluggish, meandering streams. One of their peculiar features--the sandy circular "mounds," 2 to 10 ft. high and 20 to 30 or even 50 ft. in diameter, sometimes surmounted by trees in the midst of a treeless plain and sometimes arranged in circles and on radii, and decreasing in size with distance from the centre of the field--has been variously explained. The mounds were probably formed by some gentle eruptive action like that exhibited in the "mud hills" along the Mississippi below New Orleans; but no explanation is generally accepted. The prairies shade off into the coast marshes. This fringe of wooded swamp and sea marsh is generally 20 to 30, but in places even 50 and 60 m. in width. Where the marsh is open and grassy, flooded only at high tides or in rainy seasons, and the ground firm enough to bear cattle, it is used as range. Considerable tracts have also been diked and reclaimed for cotton, sugar and especially for rice culture. The tidal action of the gulf is so slight and the marshes are so low that perfect drainage cannot be obtained through tide gates, which must therefore be supplemented by pumping machinery when rains are heavy or landward winds long prevail. Slight ridges along the streams and bayous which traverse it, and occasional patches of slightly elevated prairie, relieve in a measure the monotonous expanse. It is in and along the borders of this coast swamp region that most of the rice and much of the sugar cane of the state are grown. Long bar-like "islands" (conspicuous high land rising above the marsh and prairie)--Orange, Petite Anse, Grand Cote, Cote Blanche and Belle Isle--offer very interesting topographical and geological problems. "Trembling prairies"--land that trembles under the tread of men or cattle--are common near the coast. Most of the swamp fringe is reclaimable. The marshes encroach most upon the parishes of St Charles, Orleans and Plaquemines.