20 GEOLOGY OF ILLINOIS. 



would furnish a cheap and abundaut supply of fuel ; so that the most favora- 

 ble conditions exist here, apparently, for the prosecution of this business on an 

 extensive scale. In the northern and central portions of the county, the Bur- 

 lington limestone is the only rock that can be made available for this purpose, 

 except between Gilead and Cap au Gres, where the Trenton limestones are 

 found, a portion of which seem to afford a good material for this purpose. 

 None of the limestones of these groups, however, afford as pure a lime as some 

 of the beds of the St. Louis series, nor do they outcrop generally under such 

 favorable conditions, for the manufacture of lime. 



Glass Sand. The St. Peters sandstone, of which nearly one hundred] and 

 fifty feet in thickness is exposed at the Cap au Gres bluff, in this county, will 

 furnish an excellent white sand for the manufacture of glass, in great abund- 

 ance. No other rock in the Mississippi valley furnishes a sand for this pur- 

 pose, equal to that obtained from this formation, and at the point above men- 

 tioned, the supply of this material is absolutely inexhaustible, and the outcrop 

 is so situated, that the material could be transferred directly from the quarry 

 on to steamboats, or barges, and cheaply transferred to any point on the river, 

 where it might be desirable to establish glass manufactories. At La Salle, this 

 business is already established, and the material is obtained from an outcrop of 

 this sandstone in that county, and there is no apparent reason why the manu- 

 ture of glass should not be successfully carried on here as well as there. 



Minerals. Small pieces of the sulphuret of lead, or " galena," have been 

 found in the superficial deposits of this county, as well as in various other por- 

 tions of the State, and their discovery has led to considerable speculation as to 

 the probability of finding lead mines in this region; but, although the entire 

 thickness of the Trenton group, (the true lead-bearing formation of the North- 

 west) is well exposed here, we found no indications of its being a mineral-bear- 

 ing deposit in this portion of the State. On the contrary, it is entirely differ- 

 ent, in its lithological characters, from the lead-bearing rocks of the Northwest 

 being here a rather soft, coarse grained, yellowish gray limestone, exhibiting 

 nowhere in this region the magnesian character that every where prevails in 

 the lead producing rocks, of Lower Silurian age. It is probable the few spe- 

 cimens of galena found in this county, have been transported from the northern 

 lead mines by drift agencies, as both galena and native copper are frequently 

 found in the drift deposits in various portions of the State, and under conditions 

 that show that they have no relation with the underlying formations. Even 

 if the specimens of lead ore that are reported to have been found in the vicinity 

 of the outcrop of the Trenton limestone in this county, really came from that 

 formation, they have not indicated the presence of such an amount of lead in 

 the rock formations of this county, as would justify the expectation of their 

 affording productive lead mines. The same agency, by which boulders of 



