MCHENRY AND LAKE COUNTIES. 133 



It is just possible that I have drawn the dividing line between the Niagara 

 and Cincinnati groups too high up, and that these beds should properly be con- 

 sidered as forming the base of the upper Silurian. In referring them, however, 

 as I did, I was influenced by their resemblance to undoubted Cincinnati beds, 

 farther to the westward, as well as by the position of the outcrop. We fre- 

 quently find, moreover, in this part of the State, a greater or less similarity in 

 the beds on both sides of the line of separation of two members of the Silu- 

 rian, lying conformably one upon the other, and occasionally what appear to be 

 beds of passage between the two. 



Besides the Niagara and Cincinnati groups, which we know to underlie por- 

 tions of the territory of this district, the Galena limestone may possibly be also 

 found to occupy a very narrow strip along its northwestern border. As, how- 

 ever, I am aware of no outcrops nor exposures, whatever, of this formation, in 

 the district, and its presence here is only inferred from the facts afforded by the 

 exposures in the adjacent portion of Boone county, directly to the westward, 

 this bare mention of it may be sufficient in this report. 



Economical Geology. 



Building Materials. The only stone quarry of any extent within the district, 

 is that which has just been described under the head of the Cincinnati group, 

 a little distance east of the western line of McHenry county. The rock here 

 is generally too thin bedded and contains too much chert, to serve all purposes 

 as a building stone, but nevertheless answers well for foundations and for the 

 rougher kinds of masonry generally. In the other localities where the beds of 

 rock appear, they have been worked only to a very slight extent, and for the 

 manufacture of lime alone. It does not appear, moreover, from the nature of 

 of the rock itself, that any very good building stone will ever be obtained from 

 the most of these outcrops. In many parts of the district, the erratic boulders 

 of the Drift are used more or less in rough masonry, and in some places along 

 the Fox river, boulder quarries, so to speak, are worked in the ridges which 

 have been mentioned before, as being largely made up of loose masses of lime- 

 stone rock. 



Good clay for making brick is found in most parts of the district, although 

 in some instances the same difficulty is met with as in Cook county the clay 

 contains too large a proportion of lime or limestone pebbles to make a good arti- 

 cle. The prevailing color of the brick made in this district is red or reddish 

 brown. A white or straw-colored brick is made, however, at Woodstock, and 

 at McHenry, in McHenry county. At Woodstock the clay from which the 

 white brick is made is obtained under a peat bed, and may possibly be a sedi- 

 mentary formation, more recent than the Drift. That at McHenry I am in- 

 clined to think belongs to the Drift proper. The same clay that is used for 



