62 



PEOTOZOIC ROCKS 



profit, in the vicinity of the mines. The Lower Magnesian Limestone may, in one 

 respect, be considered more favourably situated than the Upper, as a mineral-bear- 

 ing rock. It is an established fact in Geology, that, all other things being equal, 

 the lower or older a rock is, the more likely it is to be metalliferous, because nearer 

 the sources from whence experience indicates that metallic materials find their way 

 into its recesses ; in other words, because in closer proximity to rocks of igneous 

 origin. But it has been shown that the inferior beds of the Lower Magnesian 

 Limestone of the Upper Mississippi lie at least three or four hundred feet below 

 the lead-bearing beds of the Upper Magnesian Limestone, and are separated from 

 the crystalline and igneous rocks by the lower sandstones only. 



By reference to my former Report in 1839 (Senate Doc. 407, p. 30), it will be 

 seen that it was considered a remarkable circumstance that, in a mining district so 

 rich as that south of the Wisconsin River, no basalt, greenstone, porphyry, or 

 other intrusive or crystalline rocks, had, up to the time of the survey of 1839, been 

 observed there, since these are in general found in place in the vicinity of produc- 

 tive mining districts ; but I then expressed my belief, based upon the abundance of 

 metallic lodes in that lead region, and upon irregularities in the dip of the strata in 

 some localities, that granite and trappean rocks could not be far off. This supposi- 

 tion has been fully verified by the present survey. One of the most interesting of 

 its discoveries has been the establishment of the fact that the lowest beds of F. 1, 

 previously described, rest either immediately on crystalline or trappean rocks, or 

 there intervenes but an inconsiderable thickness of metamorphic beds. 



There can now be little doubt that the whole mining region of the Mineral Point 

 and Dubuque districts of Wisconsin and Iowa, is based upon a syenitic and granitic 

 platform, which would, in all probability, be reached by penetrating to the depth of 

 from two thousand to four thousand feet. 



These facts, taken together, may be considered as favourable to the metalliferous 

 character of F. 2. Fortunately, I am able to bring several actual discoveries in 

 corroboration of this inference. 



Near the base of a bluff composed of F. 2, on the west side of the Mississippi, 

 some ten or fifteen miles above the mouth of Turkey River, and just above the 

 French village, from seven to ten thousand pounds of lead ore were obtained from 

 openings in the rock by Dr. Andros. More or less galena is found here, in all the 

 horizontal openings, for the distance of half a mile to a mile. 



Near the mouth of the Kickapoo, on the southeast quarter of Section 10, Town- 

 ship 7 north, Range 5 west, of the 4th Principal Meridian, pieces of lead ore, 

 weighing from half to three-quarters of a pound, have been obtained from cherty 

 beds of the inferior part of the Lower Magnesian Limestone. A company has 

 lately commenced exploring there, and has obtained some hundred pounds of galena. 



On the opposite side of the same valley, Hearn and Ward obtained about four 

 hundred pounds of galena; some masses weighed fifteen pounds. On Section 15, 

 Township 7 north, Range 5 west, of the 4th Principal Meridian, some lead ore 

 has been found. 



In the hills at the first great western bend of the Kickapoo, a little below the 

 mouth of Plum Creek, Hearn and Miller discovered some lead ore. 



