50 



GEOIvOGY OP IvA SAIvIvE COUNTY. 



a coal at one point may differ g-reatly from that of the 

 same bed at another place. 



A seam of coal one foot thick yields about 1,120 

 tons of coal if all is removed; one two feet thick, 2,240; 

 one three feet thick, 3,360 tons, but if pillars are left 

 to support the roof these will take about one-fourth of 

 the whole and the yield will be but three-fourths of 

 these fio-ures. The county has fuel sufficient for 

 another century at least unless the demand increases 

 much faster than it has done for the last twenty years. 



Petroleum. ==It is useless to look for pet roleum 

 in La Salle county. There are no indications of its 

 presence, and the rocks in which it is g'enerally found, 

 Upper Devonian or lower carboniferous, Niag-ara or 

 Trenton-Gincinnati shales do not occur here. 



Natural Gas. ==Gas was used for fuel and lio-ht 

 in the southeast part of the county as early as 1877 or 

 1878. It was always found proceeding- from some 

 of the beds of the thick deposits of clay which form 

 the coal measures in that part of the county. Most of 

 these discoveries were made in Allen Township. Later 

 it was found near Mendota, but in small quantity. 

 The source of this must have been below the coal area. 

 It is found in connection with coal No. 2 at Streator, 

 and, we presume, at other places, and at Streator in a 

 stratum about ten feet above No. 2 in considerable 

 quantity. But the supply is limited. The Cincinnati 

 shales, the great source are entirely, or almost entirely, 

 wanting-, and while g-as may be found from time to 

 time, we may feel pretty sure that time and mone}^ 

 spent in searching- for it are wasted. 



Salt water exists, but not of sufficient streng-th — 

 saltness — to warrant an attempt at manufacturing- 

 salt. The salt marsh six miles southwest of Ottawa 

 furnished a considerable quantity of weakly salt water, 

 and a well bored farther west a g-reater volume. 



