REPORT OF EXECUTIVE COMMITTEE. 
21 
rock, which is hard and compact, resisting the action of the 
weather, while this is more usually a crumbling mass of siliceous 
sand. In some localities it contains a considerable proportion 
of oxide of iron, which, serving as a cement, gives it greater 
compactness and durability. This is true of it to a greater 
extent in its eastern exposures than in the western part of the 
district. 
As to extent and geographical position, the Potsdam or 
Lower Sandstone almost surrounds the northern and central 
Azoic district just described, there being but two slight inter¬ 
ruptions in its continuity — one at the north-east, near the 
shore of Lake Superior, the other in the north-west, above the 
Falls of St. Croix. Its chief development is in the middle por¬ 
tion of the State, where it may be seen upon the geological 
map in the form of a crescent, its lower disc resting upon the 
Counties of Dane and Iowa, and its two horns touching the 
St. Croix and Menomonee Rivers in the north-west and south¬ 
west. The width of the centre is about one-hundred miles; of 
the horns thirty to sixty, the western being the wider of the 
two. The thickness of this formation is, in some places, five 
hundred feet; the upper portions running into alternations 
and intercalations with Magnesian Limestone; while at various 
points natural vertical sections expose argillaceous and dolo- 
mitie bands. 
Next below the Potsdam Sandstone on the map, and next 
above in the order of position we find the 
Lower Magnesian Limestone , (Calciferous Sandstone of 
Eaton.)—This is a light grey or ash-colored rock, compact 
and fine in its crystalline character, and marked by numerous 
cavities or geodes, lined with crystals of quartz, calcite, &c. 
The lower beds are frequently oolitic, (made up of rounded 
cavities filled with nodules) and in some places the entire mass 
is composed of rounded and angular concretions. For these 
reasons it weathers very irregularly, and hence is not usu¬ 
ally valuable for building purposes; though at Ilortonville, 
Outagamie County, at Ripon, and in several other localities, 
very respectable quarries have been opened. It is indicated 
