320 



GEOLOGY. 



Section in Southeastern Minnesota. 1 



Names of Formations. 



Thickness 

 in Feet. 



Characteristics. 



c Devonian 







•~ f Unconformity. 







J2 •{ Niagara(?) limestone. . 



100-150 



Light - colored crystalline limestone, often 





porous. 



Unconformity. 







c - 



Maquoketa shales .... 



75-100 



Shales; some sand, finely bedded. 



o3 



( = Hudson River) 









Galena limestone 



75-100 



Bluish or grayish, evenly bedded, varying 







from compact to vesicular; sometimes 







arenaceous and often dolomitic. 



o 



Trenton limestone. . . . 



160 



Light blue or gray; non-dolomitic. 





St. Peter's sandstone. . 



122 



White and friable. 





Shakopee limestone. . . 



75 



Irregularly bedded, cherty; concretionary and 

 brecciated layers; heavy -bedded light buff; 

 shaly layers. 



a 



New Richmond sand- 







.2 



stone 



25-40 



Coarse quartzose sandstone, sometimes inco- 

 herent; sometimes cemented. 



si 







Lower Magnesian 







u 



limestone 



St. Croix ( = Potsdam 

 sandstone) 



200 



Yellowish, dirty-appearing limestone; heavy- 

 bedded below; thin-bedded above; partly 

 arenaceous. 



Section in Central Texas. 2 



Names of Formations. 



Thickness 

 in Feet. 



Characteristics. 



jh § f Unconformity. 

 .5 "3 1 San Saba 



|l|i Leon 



315± 



440 + 



Dolomites and chert. 



Sandy shaly dolomite; siliceous magnesian 



limestone ; brown dolomite ; Burnet 



marble. 



Strata nearly horizontal. The equivalent of the Niagara is apparently absent. 

 Thicknesses given are incomplete. 



In Arkansas the system, though largely of. limestone, includes 

 thick beds of novaculite, 3 a fine pure siliceous sandstone, from which 

 whetstones are made. In the southwest, the Ordovician and Silurian 



1 Winchell, Geol. and Nat. Hist. Surv. of Minn., Vol. I, 1872-82, p. 280, and PI. X. 

 Revised to agree with nomenclature of frontispiece of Vol. VI, atlas 1900-1901, Geol. 

 and Nat. Hist. Surv. of Minn. The Shakopee, New Richmond, and Lower Magnesian 

 formations of Minnesota correspond to the Oneota of Iowa, and to the Lower Mag- 

 nesian of Wisconsin. 



2 Dumble, Geol. Surv. of Texas, 1st Ann. Rept. PI. III. 



3 Griswold, Ark. Geol. Surv. Ann. Rept. 1890, Vol. III. 



