POTSDAM SANDSTONE. 357 



OHAPTEE VII. 



LOWER SILURIAK 



POTSDAM SANDSTONE. 



There rests upon the very irregular surface of the metamorphic 

 rocks above described, filling up its depressions and for the most 

 part surmounting its hills (over the area so occupied), a deep and ex- 

 tensive deposit of sandstone, known under the above name. That it 

 is the exact equivalent of the Potsdam sandstone of JS"ew York, as 

 would seem to be implied by the name, is not absolutely certain, but 

 as the term has been used to designate this formation in previous re- 

 ports upon the geology of the state, and as the weight of evidence 

 and authority favors this view, the name Potsdam sandstone will be 

 used without further qualification in this report. 



The upper surface of this formation is essentially uniform and 

 nearly horizontal, and is overlaid by the Lower Magnesian limestone. 

 These two formations, then, the limestone above and the Archaean 

 rocks below, furnish one of the means by which the sandstone may 

 be identified and its position and thickness determined. Since its 

 upper surface is nearly uniform, and its bed very irregular, it 

 necessarily varies greatly in thickness, the known range within 

 the state being from zero to about 1,000 feet. Within our district 

 its variation in thickness is known to be but little short of this, and 

 from the nature of the case, it is evident that the extreme thickness 

 of this rock may much exceed that which has been observed. This 

 irregularity in thickness- should be kept in mind in making any cal- 

 culations dependent upon the depth of this formation. 



General Character. The rock is chiefly composed of cemented 

 grains of silicious sand. To the unassisted eye, these grains appear 

 spherical, but upon examination under the microscope, they are seen 

 to be more or less angular and irregular, and show that they have 

 been formed from small fragments, and in some cases, perhaps, 

 minute crystals of quartz, which have been worn by friction to their 

 present form. These grains vary much in size in different localities, 

 and in the various strata of the formation. 

 Wis. Sun. 17 



