LOWEE SILUEIAN SYSTEM. 175 



equal in area to about one-third of the whole of that occupied 

 by all the other Lower Silurian formations in the State. It 

 occupies large portions of both Winneshiek and Allamakee 

 counties, together with a portion of Clayton; and although 

 its southwesterly dip has carried it beneath the Galena lime- 

 stone there, yet it is again found, after passing out of sight 

 for a distance of about ten miles, in the bottom of Turkey 

 river valley, where it is seen along the greater part of its 

 entire length, having been exposed by the cutting through of 

 the Galena limestone by Turkey river in the erosion of its 

 valley. From this valley, the southwesterly dip of the 

 formation has carried it so deeply beneath the surface that it 

 is seen no more in Iowa. 



The thickness of this formation as seen along the bluffs of 

 the Mississippi is about eighty feet, but in Winneshiek 

 county we find the thickness increased to upwards of two 

 hundred feet. 



Economic Value. The greater part of the bulk of this 

 formation is worthless for economical purposes, but a con- 

 siderable portion, quite enough to meet the wants of the 

 inhabitants, affords excellent material for building purposes 

 and also for the production of common lime of excellent 

 quality. There is no lack of common building stone wherever 

 it is exposed, and in some places there are compact and 

 evenly bedded layers which afford fine material for dressing 

 into all desirable forms for use as caps, sills, etc. The best 

 quarries of this rock that have been examined are those 

 just above Dubuque, and those in and near the town of 

 Decorak. The worthless portion of the formation consists of 

 clayey shales and shaly limestone. The latter is very 

 prevalent in Winneshiek county, as seen along the valley 

 of the Upper Iowa, where the formation is also thickest. 

 There are also some beds of tolerably pure clay associated 

 with the calcareous portions which promise to be valuable. 

 One of these occurs near the town of Clermont, in Fayette 

 county, where Mr. Dibble has been manufacturing from it the 

 light-colored variety of brick, known as ''Milwaukee brick." 



