40 ECONOMICAL GEOLOGY. 



river from its mouth to the vicinity of Indian Creek, a distance of 

 about ten miles. This sandstone will furnish an inexhaustible sup- 

 ply of the best glass sand to be found in the Mississiypi Valley, and 

 in its economical importance it is second only to the Coal Measures 

 in the value of its products. It is found at only two points in the 

 State outside of LaSalle county, viz : at Grand de Tour, in Lee, and 

 at Cap au Ores in Calhoun, counties. Its maximum thickness is 

 probably about 225 feet in the southern part of this county, but it 

 thins out to the northward, so that in the northern portion it does 

 not much exceed 150 feet. Its extensive outcrops in the bluffs of 

 the Illinois river make its economical products easily accessible to 

 both railroad and water transportation, and the abundance of coal 

 to be obtained in close proximity to this sandstone indicates that 

 this is one of the most favorable points in the State, or in the United 

 States, for the manufacture of all kinds of glassware, and several 

 extensive factories of this kind have already been established in this 

 county. 



The Trenton limestone, which is the next formation above the 

 St. Peter's sandstone, has been so much eroded where it appears in 

 natural outcrops in this county, that no accurate estimate of its 

 original thickness could be made from an examination of its sur- 

 face exposures, but in boring at Streator for artesian water, its thick- 

 ness was found to be a little over 203 feet, which is not more than 

 half its average thickness in other portions of the State, where it 

 has not been subjected to erosion. The outcrops only show the 

 presence of from 25 to 75 feet of the lower part of the formation, 

 the remainder, with the overlying Upper Silurian strata which were 

 probably deposited over this portion of the State, have been removed 

 by the long-continued eroding agencies which immediately preceded 

 the Upper Carboniferous era. 



The Trenton limestone affords some good building stone, and some 

 of the layers take a good polish, and make a handsome marble. 

 Usually it has too large a per cent, of alumina and magnesia to 

 make a good material for the lime-kiln. 



Clays suitable for brick, pottery and drain tile are abundant, and 

 some of the fire-clays of the lower Coal Measures seem to be adapted 

 to the manufacture of fire-brick. 



Coal is by far the most important and valuable mineral product 

 of LaSalle county ^ and its favorable position on the extreme north- 

 ern border of the productive coal field enhances the value of this 

 product, both for consumption in manufacturing establishments at 



