LOWER SILURIAN SYSTEM. 173 



sandstone has ; because, like that formation, it appears only 

 in the bluffs and valley-sides of the same streams. It is, 

 however, a more conspicuous formation than the Potsdam 

 sandstone, because, being a firm rock, it presents bold and 

 often picturesque fronts along the valleys. If it were not for 

 the erosion of the river-valleys, the Lower magnesian lime- 

 stone, like the Potsdam sandstone, would never have been 

 seen in Iowa, and the succeeding formations now cover them 

 up to the very verges of those valleys. 



Its thickness is about two hundred and fifty -four feet, and is 

 quite uniform in composition, being a nearly pure, buff- 

 colored dolomite. It lacks a uniformity of texture and 

 stratification, which causes it to weather into rough and 

 sometimes grotesque shapes, as it stands out in bold relief 

 upon the valley- sides. 



Economic Value. Owing to this lack of uniformity in 

 texture and bedding, the rock is not generally valuable for 

 building purposes. Some parts of it, however, may be 

 selected which serve an excellent purpose for such uses, and 

 the cities of Lansing and McGregor are supplied largely from 

 this source. Near each of these places, some beds afford 

 very good material for dressing into caps, sills, etc., for 

 buildings. It has also been used to some extent for making 

 lime, but thus far, that made from the Trenton limestone near 

 Dubuque is preferred. 



Some years ago small quantities of lead ore were dis- 

 covered in this formation in the bluffs that border the valley 

 of the Upper Iowa in Allamakee county, which excited great 

 hopes of profitable mining, and the town of New Galena 

 was consequently founded there. The amount of lead ore 

 obtained there, however, proved too small for profitable 

 working and the mines were soon abandoned. From these 

 experiences, and from other information which has been 

 accumulating during many years, it is evident that there is 

 no hope of profitable lead-mining in the Lower Magnesian 

 limestone. 



Fossils. The only fossils that have been found in this 



