130 GEOLOGY OF ILLINOIS. 



such a manner as scarcely to be detected. Farther to the northward, between 

 this place and Waukegan, I noticed bands or strata of different colored clays in 

 the upper portion of the bluffs at one or two points. 



Irregular pockets of sand and gravel, sometimes with a kind of rough strati- 

 fication of the contained material, and large and small boulders of nearly all 

 kinds of rock, are scattered abundantly throughout the hard-pan and clay of 

 which the cliffs are mainly composed. One of the largest of these boulders 

 was seen on the beach at the foot of the cliffs, a little north of the southern 

 line of Lake county. The material of the mass is a light blue or drab colored, 

 close grained, impure limestone, containing a few silicified crinoidal stems, etc., 

 but not enough of fossil remains to determine the age of the beds from which 

 it was derived, though it is probably from some of the silurian rocks of Wis- 

 consin. Its dimensions I was unable to take with accuracy, as it was deeply 

 bedded in the sand and partly covered by a land-slip from above, but the 

 exposed portion was about ten feet by six or seven, on its upper surface, stand- 

 ing three or four feet above the beach. Its upper surface was polished, but 

 not level, and showed striae in nearly all directions, but with the deepest ones 

 and largest number in the direction of its greatest diameter. Other smaller 

 masses of the same rock are frequently found with two or more sides flattened 

 and striated, and it seems quite possible that this larger mass, if fully exposed, 

 might show other similar striated surfaces to the upper exposed one. Most of 

 the large boulders are of limestone, the masses of the primary or intrusive 

 rocks are generally of comparatively small size, or when of considerable size, 

 are but rarely met with. 



Passing away from the immediate vicinity of the coast, where the frequent 

 deep ravines afford an occasional view of the lower clays, we find no good sec- 

 tions of the Drift in Lake county. There are no natural exposures, and all the 

 data which can be obtained from wells, etc., are meagre and unsatisfactory; 

 they seldom penetrate more than forty feet, and but little is met with but blue 

 clay or hard pan, with an occasional pocket or irregular seam of quick sand or 

 gravel. Boulders, however, are tolerably abundant on the surface, and are also 

 met with in these excavations, many of them of considerable size and weight, 

 and of nearly every material, granite, syenite, greenstone, trap, etc., as well as 

 of the more recent sedimentary rocks, such as limestone and sandstone. In 

 the western part of the county, near the Fox river, we find the ridges, in some 

 places, to be largely composed of rolled limestone boulders. The same character 

 has been observed further south along the same stream, and remarked upon in 

 the chapter on Cook county. The material, judging from the lithological char- 

 acters and contained fossils, is chiefly derived from the beds of the Niagara 

 group, to the northward, in the State of Wisconsin. 



