128 GEOLOGY OF ILLINOIS. 



Passing still further westward in Lake county, the general appearance of the 

 country is found to be the same, undulating prairie and forest, with here and 

 there over the surface, small level prairies and lakes or ponds. These latter 

 are most numerous in the western and northwestern portions of the county 

 where they are extremely abundant and vary in extent from a few acres to sev- 

 eral square miles. The largest are those on the upper course of the Fox river, 

 near the McHenry county line, Pistakee Lake and Fox Lake, which are from 

 four to seven miles in length, and a mile or more in breadth. The others sel- 

 ' dom exceed one or two square miles in area, and vary in character from quiet 

 land-locked ponds to shallow, grassy marshes, differing but little from the ordi- 

 nary wet prairie or slough. Indeed, almost every intermediate form between 

 the two may be found in this region. The larger lakes, in many instances, are 

 themselves widely margined with a growth of wild rice and various aquatic 

 grasses and weeds, the matted stems of these, together with the floating con- 

 fervoid vegetation, forming, in some places, a mass of sufficient buoyancy to 

 support the weight of a man. When, however, this mat is once penetrated, a 

 stick or an oar may sometimes be thrust down for a depth of several feet, meet- 

 ing with scarcely any more resistance than is furnished by its own buoyancy. 

 There are in Lake county, including the smaller ones, some twenty or thirty 

 of these lakes or ponds ; their average extent is, perhaps, nearly one square 

 mile. 



Passing westward into McHenry county, we find much of the surface of the 

 same character, but also a much greater proportion of prairie, both level and 

 undulating. The wooded country becomes more broken, even rising in some 

 instances, into what may be called in this part of the country, hills of moder- 

 ate elevation.. The general characters of soil and timber continue about the 

 same; the small lakes, however, so characteristic a feature in the adjoining 

 county, are scarcely met with at all to the westward of the Fox river. The 

 prairies of this county, which, including under this head the low-lying marshy 

 tracts or sloughs, comprise probably two-thirds, or a still greater proportion of 

 its surface, show in themselves rather greater variety of soil and surface than 

 those in the counties farther to the south. We have here the gently undulat- 

 ing or rolling prairie, a continuation of that of the counties lying to the south 

 and west, with its dark brown or blackish upper soil of varying depth, with a 

 sandy or gravelly clay sub-soil, and with narrow strips of marsh or slough 

 between the undulations. This is the general character in the southern tier of 

 townships, and to a considerable extent, though less generally, in other parts 

 of the county. In the central, and in some other portions of the county, the 

 "surface of the prairie sometimes becomes less undulating, and even apparently 

 level, though still preserving sufficient rise to afford good drainage. A good 

 example of this variety of prairie surface may be well seen in the Kishwaukee 



