2o 4 THE POPULAR SCIENCE MONTHLY. 



six inches, or it may be six feet. Along the hanks of the Mississippi 

 it presents, not infrequently, perpendicular sections of six hundred 

 feet. Its firmness in maintaining its position in such exposed bluffs is 

 due to the infiltration of the cements of lime and iron while in the 

 process of deposition, or subsequently. It is more largely developed 

 along the Missouri than along the Mississippi. There are other places 

 where the surface-soil may be peaty, from the preservation of dead 

 vegetation. Extensive level tracts, that are submerged a large part 

 of the year, may present a peaty soil. Very often also in such peaty 

 places there will be found patches of highly-calcareous soil, resulting 

 from the accumulation of fresh-water shells, or from the precipitation 

 of the carbonate of lime from waters that enter the marsh from lime- 

 stone districts. 



But, whatever the character of the surface-soil, it must be borne in 

 mind that it is accidental, and is always superinduced by causes that 

 have operated since the advent of the drift. Its influence is strictly 

 superficial, rarely exceeding three feet below the natural surface. 



No. 2. We come now to consider that which lies below the sur- 

 face-soil. If we omit from this enumeration the " blufl-formation," and 

 the alluvium of other streams which sometimes has a considerable 

 thickness, we shall have two different substances, equally pertaining to 

 the drift, and occupying the same relative position in different locali- 

 ties, that claim notice : 



1. A clay subsoil. 



2. A gravel or sand subsoil. 



Now, although these are mentioned as appearing first beneath the 

 surface-soil, it must not be understood that they appear there invari- 

 ably, nor even usually. It is probably true that throughout the greater 

 portion of the Northwest they are entirely wanting, and that feature 

 of the drift prevails which will next be considered. They are men- 

 tioned here because they constitute an essential part of the drift, and 

 must not be overlooked in giving its character and composition. 



By the first, here denominated a clay subsoil, is not meant a grav- 

 elly clay, or one in which stones are present. It is, rather, a close, 

 plastic, fine clay, with little observable sand. It is impervious to 

 water, and is benefited by artificial drainage. It prevails in much of 

 Southwestern Michigan and Northwestern Ohio. It occupies a large 

 tract in Northeastern Illinois and Northwestern Indiana. It also proba- 

 bly underlies the Red River flats in Minnesota, and perhaps a belt of 

 land rudely conforming to the shore of Lake Superior at its western 

 extremity. When shafts are sunk through this clay subsoil, so as to 

 reveal its composition and arrangement, it is seen to be handsomely 

 laminated horizontally. The individual layers are separated by thin- 

 ner layers of fine sand. Those of clay are usually about two inches in 

 thickness, but may be no more than one-eighth of an inch ; the layers 

 of sand are rarely more than half an inch in thickness, and are apt to 



