LOWER SILUETAX SYSTEM. 1 / i 



formation and an increased westerly dip there. The outcrop 

 of this formation traverses portions of the counties of 

 Howard, Winneshiek, Allamakee, Fayette, Clayton, Dubuque, 

 and Jackson. It occupies a greater surface area in Clayton 

 county than in any other, but it exhibits its greatest develop- 

 ment in Dubuque county. 



It is very uniform throughout in its lithological characters, 

 being a nearly pure dolomite, with a slight admixture of 

 silicious matter. It is not very uniform in texture however, 

 which causes it to decompose unequally, and consequently to 

 present interesting forms in the abrupt bluffs of it, which 

 border the valleys. Some of these are strangely picturesque 

 and wild in their appearance, especially in the deep glens 

 and valley-sides. 



Economic Value. The want of uniformity of texture in this 

 limestone, usually renders it unfit for dressing. Sometimes, 

 however, especially near the top of the formation, the beds 

 are more uniform, and good blocks for dressing may be 

 obtained. It almost everywhere affords stone good enough 

 for all purposes of common masonry, and will supply an 

 abundance of road-materials in all the region occupied by it. 

 Lime has been manufactured from it, but outcrops of the 

 purer Trenton limestone are seldom far away from exposures 

 of this. The lime from that formation being purer and better, 

 it is of course preferred, and the Galena limestone neglected 

 for such purposes. 



The chemical composition of this rock in some parts of the 

 the formation being known to be similar to that of some 

 which has been successfully used for hydraulic lime, it is 

 thought probable that it also may prove valuable for such 

 pur-poses. 



This formation becomes one of great economic importance, 

 as being the source of the lead ore of the Dubuque lead- 

 mines. The report of Prof. Whitney upon these mines in the 

 former geological report, is so complete, that it is not thought 

 necessary here to give anything more than a brief outline of 



the mode of occurrence of the ore. Lead, in the form of the 

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