JO DAVIESS COUNTY. 51 



Sometimes these openings extend to the surface clays ; sometimes they 

 are covered by a cap rock. They often extend into the flint strata, 

 characteristic of the middle and lower portions of the Galena limestone. 

 There is often several crevices, or sets of these various openings, one 

 over the other ; often three : sometimes as many as five ; but one open- 

 ing or set of openings is usually larger than the others, and contains 

 the heaviest bodies of mineral. 



The flat sheets or flat sheet openings are similar to the vertical, both 

 as TO themselves and their modifications, except that they lay flat in the 

 rocks, parallel to their stratification, instead of standing upright. The 

 saddle-shaped openings and pitching openings are but the transition 

 openings from the vertical to the flat. These flat openings are charac- 

 teristic of the lower parts of the Galena limestone and of the under- 

 lying Blue and Bufl' limestone, and are not found extensively developed 

 in Jo Daviess county. The " green " or " calico " rock, below the flint 

 beds: the "brown rock." and the "glass rock," are characteristic of 

 the lower Galena limestone, their beds of passage into the Blue, and 

 the Blue itself. In these occur the pipe clay openings ; and in the Buff 

 limestone the " lower pipe clay opening " is found. These are flat 

 openings, filled with shaly limestone and a peculiar clay, from which 

 they take their name. These lower flat openings are also peculiar in 

 having more of the associate mineral deposits, such as tiff, blende, the 

 ores of zinc, etc.. than the upper vertical openings. 



In this connection I do not intend to say much as to the origin of the 

 lead ore in the North-west, nor to speak of the various theories as to 

 the origin and deposition of mineral deposits in general. The question 

 as to the origin of our lead, is unsettled, perhaps. J. D. WHITNEY, the 

 best living authority on the Galena lead basin, believes the galena and 

 its associate minerals were deposited in the aqueous or humid way in 

 the crevices of the rocks, and that the veins were filled from above 

 downwards. This theory supposes that the metals were held in solu- 

 tion in the waters of the primal ocean, in the form of sulphates, and 

 were deposited in crystaliue forms in the shape of the sulphurets. The 

 decomposition of organic vegetable or animal matter, throws off a sul- 

 phuretted hydrogen gas. which, acting upon solutions containing sul- 

 phates, is supposed to cause a reduction and precipitation of the metals 

 in the form of sulphurets. The decay of sea plants and the abundance 

 of organic life in the Trenton Period, is thought to have been sufficient 

 to produce the great precipitation of lead ore found in these rocks. The 

 writer argues his theory with ability, aud it may now be considered as 

 the one generally received. I hazard the suggestion, however, that 

 electrical action may have had much to do with the precipitation, crys- 

 talization and arrangement of these minerals. 



