76 GEOLOGY OF ILLINOIS. 



The Galena Limestone. This is a massive, grayish, yellowish or 

 brownish-drab colored Magnesian limestone friable and coarse-grained 

 near its union with the clays, but very solid in its lower stratification. 

 In Jo Daviess county it is estimated to be about 250 feet thick ; in this 

 county it has never been accurately measured, but is perhaps somewhat 

 thinner, as we are on the edge of the lead basin. Its heaviest outcrop 

 commences near the geographical center of the county. Thence west- 

 ward heavy ledges of it outcrop along the banks of Carroll creek almost 

 to Savanna. North of this little stream similar outcrops may be found 

 along the banks of Plum river. The former of these streams especially, 

 has cut its channel deep into this rock. Along this stream an anti- 

 clinal axis seems to run, as the rocks dip slightly in both directions from 

 the creek, and a slight upheave! must have once taken place here. 

 Along the ridge of elevation thus formed a fissure naturally would be 

 left. The frost, the rains, and the tooth of old Father Time disintegrated, 

 wore down, and gnawed away the rocks until the fissure became par- 

 tially filled. This, in process of time, formed the little valley in which 

 Carroll creek now runs. 



This is the famous " lead-bearing rock " of the North-west. The ore 

 occurs in fissures and caverns running through the rock, in the form of 

 what the miners call "sheet" and "cog," or crystalized mineral the 

 common sulphuret of lead. In the reddish clay overlying the rock, and 

 formed by the decomposition of its upper beds, "float" ore is found ; 

 never, however, in very large quantities. Mining operations have never 

 been carried on on a large scale, or on scientific principles. The " dig- 

 gings " extend for several miles north and west of the town of Mt. Car- 

 roll. The pick, spade, common windlass and bucket, are the only ma- 

 chinery in use. Little more than a livelihood has ever been made by 

 these primitive miners. For a long time it was thought a system of 

 deep mining would reveal heavy deposits of the ore. In two instances 

 companies were formed, and a considerable amount of capital invested. 

 In one instance water compelled the abandonment of the mine, and in 

 the other nothing was ever found to repay a tithe of the expenses of 

 the company. 



This surface mining will still go on as a temporary employment for 

 those whose other employments are not steady ; but no one will prob- 

 ably be found willing to spend money enough to thoroughly test a sys- 

 tem of deep mining. The deepest section of this rock, measured by me, 

 is one hundred and fifty feet, but the bottom was not exposed, and ex- 

 tended down indefinitely. 



The early writers have been treating the Galena limestone as a sepa- 

 rate system. We believe it is now coming to be regarded as a member 

 of the Trenton limestone, none of which latter rock outcrops in this 



