536 KANSAS CITY REVIEW OF SCIENCE. 



making pottery as their remains show, should not have acquired some knowledge 

 of molding. 



The great difficulty in the way of their attaining and using such knowledge 

 was the paucity of metals with which they seem to have been acquainted, native 

 copper, which is easily worked in its natural state, being the only one. It is 

 more surprising that they never discovered the use of lead — a metal so easily re- 

 duced and manipulated, and found in so many places throughout our country. 

 Even a fierce forest fire of fallen timber burning on an exposed vein of Galena 

 would reduce some of it to a malleable state, and thus attract their attention; but 

 so far as we know few if any implements or ornaments made of this metal have 

 been found among the Mound-Builders' relics. 



ENGINEERING. 



THE MISSISSIPPI LEVEES— HOW BUILT AND HOW REPAIRED. 



Above the confluence with the Ohio the Mississippi Valley has more of the 

 commonly accepted conformation of a valley, the land sloping on both sides 

 toward the river, forming a basin. But from the Ohio downward one has to go 

 miles away from the river before reaching the sides of the valley, there being 

 many thousands of square miles of land which are of an almost floor-like level, 

 only sloping gently southward toward the gulf. Over this floor the river has, 

 with its sediment, built itself a great roof-shaped ridge, and along the ridge-pole 

 of this roof, so to speak, the greatest stream in the world pursues its crooked 

 course. As water seeks the lowest level, of course when the river is high the 

 tendency is to "slop over" and flood vast regions of the river lands on either 

 side. Were it not for this elevated position of the Mississippi, the damage 

 wrought by the floods would be comparatively local in extent, but, as it is, be- 

 tween 32,000 and 37,000 square miles of alluvial territory are liable to inunda- 

 tion when not protected. Scientists tell us that within comparatively recent 

 times, geographically considered, but still long before -the discovery of America, 

 this vast alluvial ridge, together with the delta, did not exist, and the ''Father 

 of Waters " ran, a clear, limpid stream, from its source to the Gulf. But by 

 some change in the surface-covering of the country — probably the destruction of 

 the trees throughout the vast plains of the valley by the aboriginal inhabitants, 

 converting them into open prairies — the land was exposed to the attacks of the 

 waters, and the Mississippi was made a turbid stream, carrying each year hun- 

 dreds of acres in a liquid form, either to be superimposed upon the level lands 

 along its course, or to be built out into the Gulf, the fresh-water invader thus 

 conquering new areas of land from the salt sea. 



