32 PROCEEDINGS OF MADISON MEETING. 



especially those of the compact vitreous limestones of the lower Trenton, called 

 "glass-rock," analogous to the Assuring obtained by Daubree in shattering 

 pi ate -glass. 



5. That the coincidence in extent of the lead and zinc region with the *' driftless 

 area," as shown by Professors Chamberlin and Salisbury, and the absence of the 

 ores in the glaciated areas tends to show that the ores have been derived from the 

 mass of the rocks by gradual oxidation, secretion and lateral flow into the fissures 

 during the geologic ages to which the rocks were exposed to atmospheric agencies. 



6. That the chemical conditions favoring the deposit of zinc ores and of lead ores 

 appear to have been world-wide and most favorable when the ancient Carbonifer- 

 ous and Silurian limestones were laid down. 



Discussion. 



J. F. Kemp : 



Although not personally familiar with the region described, I have seen much 

 of similar deposits in southeastern Missouri and have been in close touch with Dr 

 W. P. Jenney in his work on those in Wisconsin. A fault-fissure seems the most 

 reasonable source of supply for such large bodies of ore, but it is extremely difficult 

 to detect such as Professor Blake shows. Dr Jenney states that he has found evi- 

 dence of horizontal faulting, but this is an obscure phenomenon to identify. At 

 Mine la Motte, Missouri, experience has shown vertical faults, in one case quite 

 extended, but near them the ore-bodies fade out into barren rock — the reverse of 

 what we would expect were the fault-fissures a source of supply. Eecent drilling 

 has given some reason for believing that the ore favors, in a broad way, the places 

 where low rolls or anticlines cross the faults. If the anticlines had once been more 

 marked and a source of some shattering, had then been impregnated with ore and 

 afterward subsided, a very puzzling ore-body would have resulted, with no very 

 apparent cause for the original cavities, but with strong analogies with what we 

 meet there today. The absence, however, of notable faults in Wisconsin adds 

 much to the difficulties of explanation. 



T. C. Chamberlin : 



There is undoubted evidence of some disturbance of the beds which embrace the 

 lead and zinc deposits, and, in some instances, the beds on opposite sides of a lobe 

 lie at difierent altitudes, but I have never seen evidence which positively deter- 

 mined whether this was due to faulting or flexure. In the case of flats and pitchers, 

 which constitute the notable feature of the lead-bearing deposits, there is evidence 

 that the mass of rock embraced within the flats and pitchers has settled down 

 somewhat, but this has never seemed to me to be an instance of faulting in the 

 larger sense. While beds have been bent and crushed, and while there have been 

 minor slips and settlings of the beds, I am not aware that there is any proof of 

 general faulting which affected the great series of Paleozoic beds down to the crys- 

 talline rocks. It was faulting of this kind that was chiefly in my mind in discuss- 

 ing the subject of the ore deposits in the Wisconsin reports and not those slight 

 minor dislocations which were of merely local and trivial extent. These latter dis- 

 turbances were doubtless influential factors in determining the flnal localization of 

 the ore deposits, but they could have no bearing upon the original introduction of 

 the metalliferous material unless they extended through the strata below. 



