142 GEOLOGY OF ILLINOIS. 



In most of these outcrops the maximum thickness at some places in the 

 formation was reached. This was not true, however, of the Galena 

 limestone. That deposit runs low, and its outcrops susceptible of meas- 

 urement are much below its full thickness. I shall describe these for- 

 mations in the descending order, commencing at the top. 



The Quaternary system. All the divisions of this system, are recog- 

 nized in this county. One of them, at least, is now attracting the atten- 

 tion of capitalists and scientific men in a marked degree. 1 allude to 

 the Cat-tail peat beds, the heaviest and best deposit of peat perhaps in 

 the State or in the North-west. 



Alluvium. An alluvial bottom extends along the Mississippi river, 

 from Savanna, in Carroll county, to a few miles below Fulton city, in 

 Whiteside county. It is from four to seven miles wide. It is naturally 

 divided into two parts, nearly equal in extent. There is the high table 

 lands not subject to overflow by the spring floods of the river, consist- 

 ing of sand prairies, sand banks, and occasional tracts of the richest 

 alluvial farm lands. The other half is that low, wet, marshy bottom 

 next to the river, and a chain of sloughs and marshes along the bluffs, 

 subject to overflow at every period of high water. Upon it grows an 

 enormous yearly crop of sedges arid grasses, and the heavy alluvial 

 timber belt of the Mississippi river. The sand-beds are finely stratified 

 and contain occasional boulders, and beds of well worn, unassorted 

 gravel. In one of these gravel beds, recently worked by the Western 

 Union Eailroad Company, a mass of transported rock of several 

 tons weight was unearthed. It lies at least four miles from the bluff on 

 either side of the river. The sand-ridge in which it was imbedded is 

 evidently an old Mississippi sand-bar, of more recent deposition than 

 the drift proper. How the great boulder came there, is a mystery. 

 Perhaps when the great river extended from the Illinois to the Iowa 

 bluffs, and the vast fields of ice came floating down in the colder springs 

 of a former geological epoch, some of them were freighted with boulders, 

 which, as the ice fields went to pieces, dropped to the sandy bottom of 

 the river. The lower water-soaked bottoms sometimes approach in 

 character imperfect peat marshes. The black vegetable mold covering 

 them is often many feet in thickness. It is comparatively free from 

 sand, and when reclaimed from the water is rich and fat, but too cold 

 and sour for general cultivation, until sweetened by tilling and drying. 

 Below Fulton city and on almost to Albany, and from the Maredosia 

 slough to Cordova the alluvium rises into a high, buff colored sand 

 prairie, fertile enough to produce fair crops, except in hot dry seasons, 

 when every green crop is parched and withers beneath the blaze of an 

 August sun. These sand prairies are old Mississippi sand-bars, resting 

 against the bluffs extending east from these two towns, and running 



