MCHENRY AND LAKE COUNTIES. 129 



prairie, and at one or two other places in the county. Lastly, we have the 

 before mentioned wet prairies, or sloughs, which combined, occupy a consider- 

 able area in this county. Small sloughs, varying in extent from one acre or 

 less to several hundred, are found in all parts of the county, but the largest are 

 in the northern tiers of townships. The soil of these wet prairies is generally 

 more or less peaty, varying in composition from ordinary black, swamp muck to 

 true peat ; its depth varies from one to twelve feet, and is sometimes even more. 

 The geological formations in this district comprise only the Drift, and of 

 the older rocks, the Cincinnati and Niagara groups. The latter, however, are 

 exposed at onl_y two or three points in the district, everywhere else being 

 deeply buried under the deposits of the Drift. These consist here, chiefly of 

 clay and hard-pan, with occasional beds of sand, gravel, etc., and with frequent 

 boulders scattered throughout the mass. Its depth over the whole district will 

 probably average at least seventy feet, being seldom less than that, and often 

 much deeper. The best section is aiforded along the lake shore, from Wauke- 

 gan southward, where the exposed face of the bluffs, washed by the lake waves, 

 and constantly exposed to their wearing action, presents an almost continuous 

 section of from sixty to eighty feet perpendicular, for twelve or thirteen miles. 

 In most places these bluffs appear to be entirely composed of clay and hard- 

 pan, without stratification or any horizontal arrangement whatever, except in 

 having the upper portion generally of finer material than the lower, as was 

 observed in the continuation of these same bluffs southward, and mentioned in 

 the report on Cook county. In some places, however, a kind of a rough strati- 

 fication may be seen, rarely extending any considerable distance, and often so 

 indistinct as to escape the notice of a casual observer. In the bluffs near Port 

 Clinton, I observed the variation of the beds more by observing the line of 

 springs, or the level at which the most of the moisture seemed to gather in the 

 face of the bank, and to some extent also by the same means farther to the 

 northward. At one point, a little north of the City of Lake Forest, I made 

 out the following section. As the cliff was nearly perpendicular and unscalable, 

 the thickness of the different ?jeds are merely estimates, their relative thick- 

 nesses, however, are comparatively unimportant, as at no two points were they 

 exactly the same : 



FEET. IN. 



1. Clay , 10 to 14 



2. Sand and cla^ intermixed . c -. ... 9 " 11 



3. Clay J " 1 6 



4. Sand 1 



5. Clay , 50 



I could not trace this section for more than a few rods along the face of the 

 bluffs, as the different beds appeared to run out or to graduate into each other in 



17 



