1 882.] Geology and Paleontology. 419 



above the river mouth, and no very satisfactory explanation of 

 the manner in which they are formed or of the forces forming 

 them. The mounds above spoken of are on the left bank of the 

 river, on the place of Mr. Louis Le Bourgeois, fifty-five miles 

 above New Orleans, they are about one and a-half miles back 

 from the river and just in the edge of the swamp. The largest 

 one is 40 feet in height and 144 feet in diameter, conical in shape 

 with no signs of a crater. 300 yards N. X E. from it is a 

 smaller one, 15 feet in height and 80 feet in diameter. 250 yards 

 E. N. E. is another, not more than 5 feet in height and 20 feet in 

 diameter. Formerly the large mound was entirely surrounded 

 by a circle of these small elevations, but they have been leveled 

 during the process of cultivation. The surface soil around the 

 mounds is the usual black alluvium of the valley. 



Mr. Ogden, U. S. Navy, and myself cut into the large mound 

 from the top to a depth of 18.5 feet, and found as follows: There 

 were less than two inches of vegetable mold, and the remainder 

 of the excavation was cut through a hard orange sand ; it was so 

 hard that the pick had to be used continuously ; single valves of 

 shells, apparently Corbula, were abundant as far down as we 

 went ; to a depth of ten feet the shells were mostly soft and cal- 

 careous, below that they were all silicified ; limestone concretions 

 were very abundant, though generally small ; six feet below the 

 surface there was a layer or bed of these shells, with the valves 

 separate ; this bed was three feet wide and long, and about three 

 inches thick, and immediately underneath it the sand was black ; 

 m some case rough concretions were attached to the shells. There 

 were numerous black spots about the size of buck-shot thickly 

 scattered throughout the whole extent of the excavation; under 

 the microscope these black spots proved to be aggregations of 

 sand ; we considered them probably the result of the destruction 

 of minute shells. Eight feet below the surface there was a hand- 

 ful of blue clay and sand mixed, and a little below that a handful 

 of fine gray sand. Half way down the side of the mound I 

 found the same material and appearances, and at the beginning of 

 the slope, the orange sand lay thirty inches from the surface ; 

 thirteen feet out from the bottom of the mound, it was necessary 

 to cut through forty-seven inches of alluvium to reach the orange 

 sand, and nineteen feet out it could not be found at all. 

 . About 100 yards from the mound there was a deep ditch, 

 "jj the bottom of which there was indication, in one place, of 

 the orange sand, eight feet below the surface, but I think 

 that it had been brought from a greater depth by crayfish. The 

 la rge mound is thickly covered with a growth of magnolia, iron- 

 w °od, cane and a species of wild climbing vine. During the sum- 

 mer season, as we were informed, flowers peculiar to the mound 

 are found. From the regular shape of the large mound, broken 

 only by holes dug by treasure-hunting negroes, it seems probable 



