GEOLOGY. 245 



to the bearing of lead. But, in truth, rocks of a schistose 

 character, composed of extensible layers, and devoid of 

 vertical fissures, like this blue limestone, seldom contain lead 

 ore in quantity. Phillips, in his recent geological treatise, 

 from v^hich we have already made several quotations, justly 

 remarks : ' It is not because of any peculiar chemical quali- 

 ty that limestone yields most lead ore on Aldstone Moor, but 

 because of its being a rock which has retained openness of 

 fissure. Gritstones, in many mining fields near Aldstone 

 Moor, are equally productive ; but shales, as being soft ex- 

 tensible layers, have closed up the fissures, and their crumbling 

 faces appear to have rejected the crystallizations which attach- 

 ed to the harder limestone, gritstone, and chert.' 



" These remarks apply, with force, to the fissured cliff" rock 

 of Wisconsin, compared to the softer and more slaty-structured 

 blue limestone beneath it. 



" It will also be remarked, that the designated lead region 

 is almost exclusively confined to the northern half of the cliff" 

 limestone formation of Iowa and Wisconsin ; which northern 

 half is occupied by its middle and lower beds. The upper 

 beds (lying in the southern portion of the district) do not, as 

 already intimated, furnish productive veins of lead ore. The 

 crevices in these upper beds seem to be less numerous, and 

 either empty or filled with iron ore (hydrated browTi oxide), or 

 calcareous spar (crystallized carbonate of lime), to the almost 

 entire exclusion of veins of lead. 



" It follows, from the above observations, that the mines in 

 the northern portion of the district are less likely to be pro- 

 ductive to a great depth, than those along its southern and 

 western boundaries. 



" It follows, also, that, in the southern portion of the dis- 

 trict not included by me in the productive lead region, mines 

 of value may yet be discovered, by sinking shafts through 



