THE LOWER SILURIAN ROCKS. 555 



1-OTJgMy or too indistinctly bedded to mako good building stone, but 

 the heavy beds just alluded to sometimes take on a different character, 

 becoming fino-granular, and very evenly bedded, and yielding a 

 superior cream-colored stone. Such a stone is quarried at one or two 

 points in the town of Westport, and has been used in the construction 

 of the State Hospital for the Insane, and the United States Court 

 House at Madison. 



THE UPPER OR ST. PETERS SANDSTONE. 



To the layer of sandstone which, everywhere in Wisconsin, Iowa and 

 Minnesota, is found resting upon the eroded surface of the Lower 

 Magnesian, Dr. Owen gave the name of "Upper" sandstone, to dis- 

 tinguish it from the " Lower " or Potsdam sandstone. He also desig- 

 nated it as the " St. Peters " sandstone, from its prominent exposures 

 on the St. Peters river in Minnesota. Mr. Hall regards the St. Peters 

 sandstone as the equivalent of the Chazy limestone of New York, on 

 account of its strati graphical position. As to the correctness of this 

 reference, I have not the means of forming an accurate opinion; cer- 

 tainly, however, between the periods of deposition of the Lower Mag- 

 nesian and St. Peters, there was a long gap, whose record is fo\ind in 

 the eroded surface of the first-named formation. 



For a purely silicious sandstone, and one only 20 to 100 feet in 

 thickness, the St. Peters has an extraordinarily wide distribution. 

 It is known in Wisconsin at points 250 miles apart from east to west, 

 and 180 from south to north, whilst to the west, south; and east it ex- 

 tends far beyond the limits of the state. In the last named direction 

 it is known to extend, because a number of Artesian wells at points 

 along the shore of Lake Michigan show it with an imusual thickness. 

 To the westward, in Minnesota, it is recognized for an additional dis- 

 tance of at least 100 miles, whilst to the southward also it is known 

 to extend 100 miles beyond the Wisconsin line. Throughout all this 

 large region, there is no question whatever of the identity of the 

 formation, or of its actual continuity. Moreover, as far south as Mis- 

 souri, the St. Peters is in all probability represented by the upper- 

 most of the alternations of sandstone and limestone that form a largo 

 portion of the Lower Silurian strata of that section. Thus it appears 

 that a nearly purely silicious sandstone of inconsiderable thickness 

 has an unbroken extent over a region whose diameters are 500 and 

 400 miles. 



In the Central Wisconsin district tlie St. Peters has never a very 

 wide surface extent, forming more commonly narrow bands around 

 the areas of the Trenton limestone. It is wholly confined to Colum- 



