626 W. H. TWENHOFEL AND F. T. THWAITES 



only fossils which have been seen. The thickness varies from 40 

 to 50 feet. 



5. The upper greensand is similar in its characteristics to the 

 lower greensand, but fossils are not nearly so common. Near the 

 top are thin layers (i inch maximum thickness) of very dolomitic, 

 fine-grained, yellow sandstone interbedded with highly glauconitic 

 medium-grained sandstone. Iron pyrite has been observed locally. 

 The thickness varies from 54 to 70 feet. 



Conditions of origin. — The Franconia sediments are wholly of 

 marine origin, a conclusion proved by the quite general abundance 

 of marine fossils. The cross-lamination everywhere present shows 

 that the deposition took place above wave-base. The much smaller 

 dimensions of the grains of sand and the higher percentages of 

 angular grains suggest that the waters were considerably deeper 

 than were those in which the Dresbach sediments of these quad- 

 rangles were deposited. The mud cracks in the lower greensand 

 prove that at times during the deposition of that member several 

 hundred square miles of the two quadrangles lay above water-level. 

 The writers believe that somewhat similar shore conditions obtained 

 during Franconia time as during Dresbach, with the difference 

 that the Franconia sediments were deposited somewhat farther 

 from land influences. Any interpretation must take into consider- 

 ation the occurrence and abundance of greensand. This evidence 

 has not been thoroughly evaluated, but nothing has been dis- 

 covered which is not in harmony with the foregoing interpretation. 



THE ST. LAWRENCE FORMATION 



The St. Lawrence formation was defined by N. H. Winchell as 

 the St. Lawrence limestone.' Its limitations were not definitely 

 given because they were not known. The stratigraphic boundaries 

 as they exist in western Wisconsin were worked out by Ulrich, and 

 in this article the term is used with the significance given it by 

 him. The strata of the two quadrangles which are referred 

 to the St. Lawrence by the writers were designated the Sparta 

 shales by Shipton on the supposition that they represented an 



' N. H. Winchell, Minn. Geol. and Nat. Hist. Surv., Second Ann. Kept. (1874), 

 pp. 152-55; Fourth Ann. Rept. (1876), pp. 32-34. 



